Question 1

Sidereal day — definition and comparison with the solar day Which statements correctly describe a sidereal day?

Accepted Answer

Introduction / Context: Timekeeping in astronomy distinguishes between rotation with respect to the Sun (solar day) and with respect to distant stars (sidereal day). Surveyors and astronomers use sidereal time for accurately predicting star positions for observations and instrument alignment. Understanding how a sidereal day compares to the solar day is essential for precise scheduling and pointing. Given Data / Assumptions: Earth rotates on its axis while also orbiting the Sun. “Ordinary” solar day means the mean solar day used in civil timekeeping. Time can be expressed in standard hours, minutes, seconds. Concept / Approach: A sidereal day measures Earth’s rotation relative to the “fixed” stars. Because Earth advances a small angle along its orbit each day, it must rotate a bit more to bring the Sun back to the same meridian, making the solar day slightly longer than the sidereal day. Numerically, a sidereal day is about 23 h 56 m 4 s, roughly 4 minutes shorter than the mean solar day of 24 hours. Step-by-Step Solution: Define sidereal day: one full rotation relative to distant stars.Compare durations: sidereal day ≈ 23 h 56 m 4 s; solar day = 24 h by definition.Conclude: the sidereal day is slightly shorter and is expressed in conventional time units. Verification / Alternative check: Star transit timings and astronomical almanacs confirm the ~4-minute difference, accumulating to ~1 additional sidereal day per year relative to solar days. Why Other Options Are Wrong: Each of (a)–(c) is correct, making “All of the above” the appropriate choice. “None of these” contradicts established astronomical practice. Common Pitfalls: Confusing apparent solar time with mean solar time; assuming both days are exactly equal; overlooking that this difference drives the drift of star-rise times night-to-night. Final Answer: All of the above

Question 2

Fundamentals of meridians in surveying and geodesy Which definition best describes the meridian of a place on the Earth, considering geographic (terrestrial) meridians used for mapping and navigation?

Accepted Answer

Introduction / Context: Meridians are fundamental reference circles used in geography, geodesy, and surveying to define longitudes and directions. Choosing the correct definition clarifies how positions are referenced on maps and how celestial observations are related to Earth-fixed directions. Given Data / Assumptions: We refer to terrestrial (geographic) meridians on the Earth’s surface. Meridians are used to define longitude and great-circle directions north–south. Terminology distinguishes between a full great circle (meridian) and its halves (individual “meridians” used in some celestial contexts). Concept / Approach: On a sphere, a meridian is the great circle that passes through the Earth’s rotation poles. Every location on Earth lies on a unique meridian. While some celestial-sphere definitions describe the local meridian as a semicircle through the zenith terminating at the poles, the terrestrial meridian in mapping is the entire great circle through both poles and the place. Step-by-Step Solution: Identify the Earth-based definition used in surveying and mapping.A meridian must pass through the place and both poles → great circle criterion.Hence option (a) matches the standard geographic definition most precisely. Verification / Alternative check: Longitude is measured along the equator from a prime meridian; each longitude corresponds to a specific great circle through both poles—confirming the great-circle definition. Why Other Options Are Wrong: (b) A plane perpendicular to the rotation axis is the equatorial plane, not a meridian plane. (c) A semicircle describes only half of the great circle; while used in celestial context, the terrestrial definition is better stated as the full great circle. (d) “Perpendicular to the equator” is true for any meridian arc, but the wording as an “arc” is incomplete compared with the standard great-circle definition. Common Pitfalls: Confusing terrestrial meridians with the observer’s “local meridian” on the celestial sphere; the former is a full great circle on Earth, the latter is often described as a semicircle through the zenith. Final Answer: A great circle passing through the place and through both geographic poles

Question 3

Homologous points in stereophotogrammetry In aerial photogrammetry, what is meant by a homologous point?

Accepted Answer

Introduction / Context: “Homologous points” are fundamental to stereoscopy and photogrammetric measurements of height and plan position. Matching these points accurately across overlapping photos enables parallax measurement and 3D reconstruction. Given Data / Assumptions: Stereo pair: two overlapping aerial photographs of the same terrain. Each ground point ideally appears on both photos within the overlap region. Interior and exterior orientation are used to relate image points to ground coordinates. Concept / Approach: A homologous point is the corresponding image of the same ground object on each of the two photographs that form the stereoscopic pair. Identifying homologous points allows the calculation of parallax differences, which are then used to derive elevations and to build models in analog or digital stereo plotters. Step-by-Step Solution: Define correspondence: the two image points represent one ground point captured from two camera stations.Use their measured parallax to compute relative height or to perform bundle adjustment.Therefore option (a) correctly captures the definition. Verification / Alternative check: In digital photogrammetry, feature matching (e.g., tie points) operationalizes the search for homologous points to densify point clouds for surface reconstruction. Why Other Options Are Wrong: (b), (c), (d) are special points on a single photo, not a pairwise correspondence. (e) Nodal points belong to lens geometry and are not “homologous points”. Common Pitfalls: Confusing homologous points (photo-to-photo) with the relation between a ground point and its single photo image. Final Answer: The image of the same ground point appearing on each photograph of a stereo pair

Question 4

Homologous points – identify the correct description Which statement correctly describes homologous points in a stereoscopic pair of aerial photographs?

Accepted Answer

Introduction / Context: Correctly identifying homologous points is the cornerstone of stereo compilation, height determination, and aerotriangulation. This question checks the conceptual definition applied in practical stereoscopic analysis. Given Data / Assumptions: Two overlapping aerial photos form a stereo pair. Each ground point within overlap appears twice, once in each photo. Parallax arises due to different camera positions. Concept / Approach: Homologous points are the corresponding image points on the left and right photographs that represent the same physical feature on the ground. Their horizontal displacement along the flight direction (parallax) is exploited to recover elevation through photogrammetric formulas and model intersections. Step-by-Step Solution: Identify the nature of correspondence: one real-world feature → two photo images.Reject single-photo special points (principal, nadir) and lens nodal points.Select (c) as the correct descriptive statement. Verification / Alternative check: Stereoplotters and modern dense-matching algorithms automatically search for homologous points to reconstruct 3D point clouds, validating the definition. Why Other Options Are Wrong: (a) Opposite corners of a single photo are unrelated to stereo correspondence. (b) Nodal points are optical-system constructs, not image correspondences. (d) Plumb points (nadir) are unique points per photo; they are not “correspondences” of arbitrary terrain features. Common Pitfalls: Assuming a ground-photo pair constitutes “homologous” points; the term applies to two photos, not ground vs. photo. Final Answer: They are corresponding images of the same ground point on the two overlapping photographs

Question 5

Perspective effects on horizontal photographs (central projection) For photographs formed by central (perspective) projection, which statements are true for “horizontal photographs” considered as perspective views?

Accepted Answer

Introduction / Context: Central projection introduces perspective effects. Understanding which sets of lines appear parallel and which converge helps interpret imagery, locate vanishing points, and perform rectification when planimetric measurements are required. Given Data / Assumptions: A camera forms images by central projection. “Horizontal photograph” here means the image plane is nominally vertical to the viewing direction, producing a perspective view of a horizontal scene. We discuss geometric tendencies, ignoring lens distortion and terrain undulations. Concept / Approach: In perspective projection, sets of lines that are parallel in space but not parallel to the image plane generally appear to converge toward a vanishing point. However, lines in object space that are themselves parallel to the image plane project as parallel lines. These principles explain why road edges meet at a vanishing point in photographs while lines lying in planes parallel to the image remain parallel. Step-by-Step Solution: Assess (a): Perspective projection makes many parallel sets appear non-parallel → true.Assess (b): Classic example—receding road edges converge → true.Assess (c): When an object’s line set is parallel to the image plane, projectors intersect the plane without angular divergence → lines remain parallel → true.Therefore, “All of the above”. Verification / Alternative check: Projective geometry textbooks derive vanishing points from directions not parallel to the image plane; empirical inspection of photographs confirms these behaviours. Why Other Options Are Wrong: Each of (a)–(c) is correct under central projection; hence (d) is the inclusive correct choice. “None of these” is inappropriate. Common Pitfalls: Confusing orthographic (parallel) projection with perspective projection; only the latter produces vanishing points. Final Answer: All of the above

Question 6

Definition of the true/rational/celestial horizon What is the plane that is perpendicular to the zenith–nadir (vertical) line and passes through the Earth’s center called?

Accepted Answer

Introduction / Context: Several equivalent terms exist in astronomy and geodesy for the reference plane perpendicular to the observer’s vertical. Knowing these synonyms avoids confusion when consulting different texts or instrument manuals. Given Data / Assumptions: Zenith–nadir line defines the local vertical at the observer’s position. The reference plane is normal to this vertical and conceptually passes through the Earth’s center. We focus on naming equivalence used in surveying astronomy. Concept / Approach: The plane perpendicular to the observer’s vertical and passing through Earth’s center is known variously as the rational horizon, the true horizon, or the celestial horizon. All three terms refer to the same fundamental plane used to measure altitudes and define horizon coordinates. Step-by-Step Solution: Identify the geometric construction: plane ⟂ to vertical through Earth’s center.Recognize the accepted synonyms in surveying–astronomy literature.Therefore, all listed names are correct. Verification / Alternative check: Star altitude is measured above this horizon plane; it is the basis for altitude–azimuth systems across references, confirming terminological equivalence. Why Other Options Are Wrong: Each individual label is accurate; hence the comprehensive choice “All of the above” is correct. “None” is incompatible with standard terminology. Common Pitfalls: Misusing “apparent” or “sensible” horizon, which involve atmospheric refraction and observer’s eye height; those are different concepts from the rational/true/celestial horizon. Final Answer: All of the above

Question 7

Limiting ground gradient for locating a triangulation baseline on even slopes For practical baseline layout in geodetic/survey work on evenly sloping terrain, which limiting gradient is generally recommended so that reductions to horizontal and meas…

Accepted Answer

Introduction / Context: A triangulation baseline should be measured with the highest possible precision because it controls the scale of an entire triangulation network. On sloping ground, choosing an alignment with a mild gradient helps reduce systematic errors and simplifies reductions to the horizontal. Given Data / Assumptions: The ground is evenly sloping; alignment choice is flexible within a corridor. Instruments and tape/EDM measurements require corrections for slope, temperature, and tension. Good practice targets small slope corrections to minimize uncertainty. Concept / Approach: When selecting or setting out a baseline, surveyors prefer gentle grades to minimize the magnitude of the slope correction and to simplify staging of tripods or invar tapes. A commonly cited limiting gradient is about 1 in 12, which balances practical site constraints with the need for precision. Step-by-Step Solution: Evaluate candidate gradients in terms of slope correction magnitude and setup stability.Milder gradients (1:12) produce smaller vertical components and smaller correction terms than steeper choices (1:8, 1:6).Thus, 1 in 12 is generally recommended as a limiting value on evenly sloping ground. Verification / Alternative check: Historic triangulation manuals emphasize gentle grades or use of stepped supports to approximate horizontal measurements; values near 1:12 recur as practical limits in many references. Why Other Options Are Wrong: (b), (c), (d) are progressively steeper, increasing corrections, instrument levelling difficulty, and setup time. (e) is inapplicable because a standard guidance exists. Common Pitfalls: Assuming any slope can be handled equally by EDM: while EDM can reduce some burdens, control of instrument geometry and atmospheric corrections still benefits from gentler alignments. Final Answer: 1 in 12

Question 8

Celestial poles and the Earth’s polar axis – basic identifications Which set of statements correctly identifies the ends of the Earth’s polar axis and their intersections with the celestial sphere?

Accepted Answer

Introduction / Context: Understanding the Earth’s axis and the corresponding celestial poles is essential for astronomical navigation, time determination, and surveying astronomy. These definitions underpin the concept of declination and hour angle coordinates. Given Data / Assumptions: The Earth rotates about a fixed polar axis in space. The celestial sphere is an imaginary sphere onto which celestial objects are projected. Extensions of the Earth’s axis intersect the celestial sphere at two points. Concept / Approach: The geographic north and south poles are the surface points where the rotation axis meets the Earth. Extending this axis to the celestial sphere defines the north celestial pole and south celestial pole. These fixed points define the axis about which the celestial sphere appears to rotate. Step-by-Step Solution: Relate terrestrial poles to the axis endpoints on Earth.Extend the axis outward to intersect the celestial sphere, naming the intersection points accordingly.All four statements align with standard astronomical definitions. Verification / Alternative check: Polaris (the North Star) lies near the north celestial pole, providing a convenient reference for azimuth determination in the northern hemisphere. Why Other Options Are Wrong: Each of (a)–(d) is individually correct, so the inclusive option (e) is the only comprehensive correct choice. Common Pitfalls: Mixing up the geographic poles with magnetic poles; the latter are defined by Earth’s magnetic field, not by the rotation axis. Final Answer: All of the above

Question 9

Length of a parallel between two meridians at latitude λ For a sphere of radius R, the length of a parallel (small circle) at latitude λ between two meridians differing by Δlongitude (in radians) equals R * Δlongitude multiplied by which trigonometr…

Accepted Answer

Introduction / Context: Map distances along parallels depend on latitude because parallels are smaller circles than the equator. Surveyors and navigators routinely apply the cosine factor to convert longitude differences into arc lengths at a given latitude. Given Data / Assumptions: Spherical Earth of radius R (for simplicity). Longitude difference expressed in radians, Δλ. Latitude of the parallel is λ. Concept / Approach: The radius of the parallel at latitude λ is R * cos λ. Therefore, an angular separation Δλ (radians) along that parallel corresponds to an arc length of (R * cos λ) * Δλ. This introduces the cosine-latitude factor into many geodetic and cartographic formulas. Step-by-Step Solution: Compute effective parallel radius: r_parallel = R * cos λ.Arc length along the parallel: s = r_parallel * Δλ.Thus, s = R * Δλ * cos λ → the required factor is cos λ. Verification / Alternative check: At the equator (λ = 0°), cos λ = 1, so the parallel is the equator with full radius R, as expected. Near the poles, cos λ → 0, so east–west distances per degree of longitude shrink toward zero—consistent with convergence of meridians. Why Other Options Are Wrong: sin λ, tan λ, cot λ, sec λ do not describe the radius of the parallel; only cos λ scales the equatorial radius to the latitude circle. Common Pitfalls: Using degrees directly without converting Δlongitude to radians; always convert to radians when computing arc length with s = R * θ. Final Answer: cos λ

Question 10

Spherical triangles – identify the incorrect statement In a spherical triangle on the surface of a sphere, which one of the following statements is incorrect?

Accepted Answer

Introduction / Context: Spherical trigonometry governs angles and sides of triangles drawn on a sphere. Unlike plane triangles, spherical triangles have curved sides (great circle arcs), and their geometric properties differ in key ways, which surveyors and astronomers must remember for accurate computations on the celestial sphere and in geodetic work. Given Data / Assumptions: Triangle lies on a sphere and sides are measured as central angles (radians or degrees). Angles are dihedral angles between great circles at the triangle’s vertices. Right angle = 90°; π radians = 180°. Concept / Approach: In a spherical triangle, each interior angle is strictly between 0° and 180°. The sum of the three angles exceeds 180° (two right angles) by an amount called the spherical excess and is strictly less than 540° (six right angles). There are also special relational properties connecting sums of sides with sums of opposite angles. Step-by-Step Solution: Check option (a): Each spherical angle < 180° → true.Check option (b): Sum of angles = 180° → false for spherical triangles; true only for plane triangles.Check option (c): Angle sum lies between 180° and 540° → true.Check option (d): If a + b = π, then A + B = π → a known spherical identity → true.Check option (e): Triangle inequality on a sphere (each side < sum of other two) → true. Verification / Alternative check: The spherical excess E satisfies A + B + C = π + E (in radians) with E > 0 for any non-degenerate spherical triangle. Hence equality to 180° cannot hold. Why Other Options Are Wrong: (a), (c), (d), (e) are valid properties of spherical triangles. (b) is the planar-triangle rule and is therefore incorrect on a sphere. Common Pitfalls: Transferring plane-triangle rules directly to spherical geometry; forgetting that spherical excess makes the angle sum greater than 180°. Final Answer: The sum of the three angles is equal to two right angles (exactly 180°)

Question 11

Solar parallax correction in altitude – select the correct expression For the Sun, what geocentric parallax in altitude should be added to the observed altitude (approximate horizontal parallax ≈ 9″)?

Accepted Answer

Introduction / Context: Parallax correction converts a topocentric (observer-based) altitude to a geocentric one by accounting for the observer’s displacement from Earth’s center. For the Sun, the horizontal parallax is small (about 8.8–9.0 arcseconds), yet the correct functional form with altitude matters in precise astronomical reductions. Given Data / Assumptions: Small-angle approximation suitable for solar observations. Horizontal parallax of the Sun HP ≈ 9″. α denotes observed altitude of the Sun. Concept / Approach: Parallax in altitude = HP × cos(altitude). This arises because the component of the Earth-center to observer displacement perpendicular to the line of sight scales with cos α. The correction is added (positive) to the observed altitude to arrive at the geocentric altitude. Step-by-Step Solution: Write correction: p_alt = HP * cos α.Insert HP ≈ 9″ → p_alt ≈ 9″ cos α.This is added to the observed altitude to reduce to the Earth’s center. Verification / Alternative check: At the horizon (α ≈ 0°), cos α ≈ 1, so parallax ≈ HP (maximum). At the zenith (α ≈ 90°), cos α ≈ 0, so parallax ≈ 0; this matches physical intuition. Why Other Options Are Wrong: sin α, tan α, cot α, sec α do not match the geometric projection of the Earth-center offset onto the line of sight. Common Pitfalls: Confusing sign conventions or using sine instead of cosine; forgetting that solar parallax is small but non-zero. Final Answer: 9″ cos α

Question 12

Great circles on the celestial sphere – identify the one through zenith, nadir, and the poles Which great circle passes through the observer’s zenith, nadir, and the celestial poles?

Accepted Answer

Introduction / Context: Great circles define fundamental reference planes on the celestial sphere. Recognizing which points each passes through is essential for interpreting transits, azimuths, and altitude measurements in astronomical surveying. Given Data / Assumptions: Zenith and nadir are the upward and downward vertical directions. Celestial poles define Earth’s rotation axis on the celestial sphere. Concept / Approach: The observer’s meridian is the north–south great circle through the zenith and nadir that also intersects the celestial poles. While any vertical circle also passes through zenith and nadir, only the meridian additionally goes through the poles; the prime vertical goes through east and west points instead of the poles. Step-by-Step Solution: List defining points for each candidate.Meridian: zenith, nadir, and poles → matches the requirement.Prime vertical: zenith, nadir, east, west → does not include poles.General vertical circle: may not pass through poles. Verification / Alternative check: Standard celestial diagrams show the meridian orthogonal to the prime vertical, both passing through zenith and nadir; only the meridian includes the poles. Why Other Options Are Wrong: Vertical circle (general) lacks poles. Prime vertical lacks poles (passes through E and W). “None” is invalid because the meridian satisfies the condition. Common Pitfalls: Confusing prime vertical with the meridian; assuming all vertical circles pass through poles (they do not). Final Answer: Meridian

Question 13

Definition of a nautical mile – choose the correct geodetic meaning A nautical mile is defined as the length of:

Accepted Answer

Introduction / Context: The nautical mile underpins marine and aeronautical navigation. Its geodetic basis links linear distance to angular measures on the Earth, facilitating direct conversion between charted angles and distances. Given Data / Assumptions: Earth approximated by a reference ellipsoid or sphere for the definition. One minute of arc = 1/60 of a degree. Concept / Approach: By historical and practical convention, a nautical mile corresponds to the length of one minute of arc of latitude along a meridian. This avoids longitude convergence issues (longitude minute varies with latitude) and gives a near-constant distance standard (modern value fixed internationally at 1852 m). Step-by-Step Solution: Identify which angular minute has near-constant ground length.A minute of latitude along a meridian provides a consistent standard → nautical mile. Verification / Alternative check: Modern definition: 1 nautical mile = 1852 m; this closely matches the mean length of 1 minute of latitude on Earth. Why Other Options Are Wrong: Minute or degree of longitude vary with latitude (zero at poles). One degree of latitude is 60 nautical miles, not one. Equatorial radius is a length constant, not an angular-based unit. Common Pitfalls: Assuming 1 minute of longitude equals a nautical mile at all latitudes; true only at the equator. Final Answer: 1 minute of latitude

Question 14

Find the declination from meridian altitudes of a circumpolar star A circumpolar star has meridian altitudes of 70° (upper culmination) and 10° (lower culmination), and both culminations occur north of the zenith. What is the declination of the star?

Accepted Answer

Introduction / Context: For circumpolar stars (declination same sign and sufficiently large relative to latitude), upper and lower culmination altitudes on the meridian provide direct equations linking site latitude and star declination. This is a common reduction in positional astronomy and astro-surveying. Given Data / Assumptions: Upper culmination altitude hup = 70°. Lower culmination altitude hlow = 10°. Both culminations are north of the zenith → star north of zenith implies δ > φ (northern declination larger than latitude). Let φ = latitude, δ = declination (both positive in N hemisphere). Concept / Approach: For a circumpolar star with δ and φ both north and δ > φ: hup = 90° − δ + φ and hlow = 90° − φ − δ. Solve these two linear equations for φ and δ. Step-by-Step Solution: Use hup: 90 − δ + φ = 70 ⇒ −δ + φ = −20 ⇒ φ − δ = −20.Use hlow: 90 − φ − δ = 10 ⇒ −φ − δ = −80 ⇒ φ + δ = 80.Add the two: (φ − δ) + (φ + δ) = −20 + 80 ⇒ 2φ = 60 ⇒ φ = 30°.Substitute: φ + δ = 80 ⇒ 30 + δ = 80 ⇒ δ = 50°. Verification / Alternative check: Check both altitudes with φ = 30°, δ = 50°: hup = 90 − 50 + 30 = 70°; hlow = 90 − 30 − 50 = 10° → matches given data. Why Other Options Are Wrong: Options 80°, 70°, 60°, 40° do not satisfy the pair of equations for the given heights; only 50° does. Common Pitfalls: Using formulas for the δ < φ case or forgetting the sign convention (north positive); misinterpreting “north of zenith,” which here implies δ > φ. Final Answer: 50°

Question 15

Right spherical triangle (C = 90°) – choose the correct identity In a spherical triangle ABC right-angled at C, which identity is correct for sin b?

Accepted Answer

Introduction / Context: Right spherical triangles are central to many astro-survey calculations (e.g., azimuth–hour angle–latitude relationships). Napier’s rules and specialized identities simplify general spherical trigonometry when one angle (here C) is 90°. Given Data / Assumptions: Triangle ABC with C = 90°. Sides a, b, c are opposite angles A, B, C respectively. Concept / Approach: Using right spherical triangle relations, the sine rule reduces to convenient forms: sin a = sin A * sin c and sin b = sin B * sin c when C = 90°. These follow from general relationships with sin C = 1. Step-by-Step Solution: Start from the spherical sine rule: sin a / sin A = sin b / sin B = sin c / sin C.With C = 90°, sin C = 1 → sin c / 1 = sin c.Therefore sin b = sin B * sin c (and similarly sin a = sin A * sin c). Verification / Alternative check: Cross-check with Napier’s rules for right triangles; the same identity emerges consistently. Why Other Options Are Wrong: They mix cosines or tangents in forms that do not correspond to the sine rule simplification for C = 90°. Common Pitfalls: Swapping side–angle correspondences or applying plane trigonometric identities in place of spherical ones. Final Answer: sin b = sin c * sin B

Question 16

Relative strength of solar and lunar tides – choose the closest ratio What is the approximate ratio of the solar tidal force to the lunar tidal force acting on the Earth?

Accepted Answer

Introduction / Context: Tidal forces arise from differential gravitational attraction across the Earth. Although the Sun is vastly more massive than the Moon, the Moon is much closer; tidal force scales with mass divided by distance cubed, so proximity dominates. Given Data / Assumptions: Tidal force F ∝ M / R^3 for a perturbing body of mass M at distance R. Use mean values for the Sun and Moon to obtain an order-of-magnitude ratio. Concept / Approach: The ratio (solar/lunar) ≈ (Msun / Rsun^3) / (Mmoon / Rmoon^3). Substituting average astronomical values yields a ratio close to 0.46. Therefore the Moon raises tides about twice as effectively as the Sun; spring tides occur when their effects reinforce each other. Step-by-Step Solution: Write ratio: (Fsun/Fmoon) = (Msun / Rsun^3) * (Rmoon^3 / Mmoon).Insert mean values (conceptually): enormous Msun but much larger Rsun drives the ratio below 1.The accepted practical value is ≈ 0.46 (about 46%). Verification / Alternative check: This value is consistent with observed spring–neap tide amplitudes and with standard geophysics texts; the solar tide is roughly 45–50% of the lunar tide. Why Other Options Are Wrong: 0.23 underestimates the solar contribution. 0.75 and 1.00 overestimate; the Sun’s larger distance reduces its tidal effect below the Moon’s. 2.20 is physically implausible (would imply solar tide bigger than lunar). Common Pitfalls: Confusing force proportionality (1/R^2) with tidal force proportionality (1/R^3); using mass only without distance-cubed scaling. Final Answer: Approximately 0.46

Question 17

Astronomical north, bearing, and azimuth – pick the correct combined statement Which of the following statements about astronomical directions and bearings is correct?

Accepted Answer

Introduction / Context: In astronomical and geodetic surveying, the local horizon system uses true (astronomical) north as a reference. Bearings and azimuths derived from the celestial sphere ensure consistent orientation over large distances and time. Given Data / Assumptions: True north is defined by the direction of the celestial pole on the horizon. Bearings/azimuths are measured clockwise from true north in practical surveying contexts. Concept / Approach: The astronomical north direction points toward the projection of Earth’s axis (celestial pole). The astronomical bearing (often termed azimuth) of a line is the angle at the observer between true north and the line, measured in the horizontal plane. Step-by-Step Solution: Confirm definition of astronomical north → direction toward the celestial pole on the horizon.Define astronomical bearing → horizontal angle from true north to the line.Recognize common terminology → azimuth is the same concept in this context. Verification / Alternative check: Standard surveying texts equate astronomical bearing with azimuth referenced to true north (as opposed to magnetic bearings referenced to magnetic north). Why Other Options Are Wrong: Since (a), (b), and (c) are all correct, “None of these” is incorrect; the comprehensive answer is “All of the above”. Common Pitfalls: Confusing magnetic north with astronomical (true) north; mixing quadrant bearings with whole-circle azimuths. Final Answer: All of the above

Question 18

Definition of a sidereal day – identify the correct reference A sidereal day is the time interval between two successive upper meridian transits of:

Accepted Answer

Introduction / Context: Time scales in astronomy are defined against different celestial references. The sidereal day anchors Earth’s rotation to the fixed stars rather than to the Sun, giving a slightly shorter day than the solar day. Given Data / Assumptions: Upper transit = culmination across the local meridian. The first point of Aries (γ) is the reference direction of the vernal equinox on the celestial sphere. Concept / Approach: A sidereal day is the elapsed time between successive transits of the vernal equinox across the observer’s meridian. Because Earth orbits the Sun, the mean Sun lags behind the fixed-star frame, making the solar day longer (~24 h) than the sidereal day (~23 h 56 m 4 s). Step-by-Step Solution: Identify the appropriate reference point for sidereal timing → the first point of Aries.Reject solar-based definitions which correspond to apparent or mean solar days. Verification / Alternative check: Star catalogs and timekeeping standards define local sidereal time as the hour angle of the first point of Aries; its return to the meridian marks one sidereal day. Why Other Options Are Wrong: Mean Sun defines the mean solar day. First point of Libra is opposite Aries and not the standard reference. Polaris is near the pole and not used to define day length. The Moon’s motion is not used to define daily time standards. Common Pitfalls: Thinking “sidereal” means any star, when the formal definition uses the vernal equinox direction for consistency. Final Answer: The first point of Aries

Question 19

Hour angle – choose the correct angular definition The angle between the observer’s meridian and the declination circle of a celestial body, measured at the celestial pole, is known as:

Accepted Answer

Introduction / Context: Celestial coordinates come in pairs: equatorial coordinates (right ascension and declination) and horizontal coordinates (azimuth and altitude). The hour angle complements right ascension by measuring how far a body has moved westward from the local meridian. Given Data / Assumptions: Declination circle: the great circle through the celestial poles and the body. Observer’s meridian: the great circle through the poles and the zenith/nadir. Concept / Approach: The hour angle H is the spherical angle at the celestial pole between the local meridian and the object’s declination circle. It increases westward with time, completing 24 h (360°) per sidereal day. Step-by-Step Solution: Identify the two great circles: meridian and declination circle of the body.The angle between them at the pole is, by definition, the hour angle. Verification / Alternative check: Local sidereal time equals the hour angle of the first point of Aries; this operational rule uses the same angle definition at the pole. Why Other Options Are Wrong: Azimuth is measured in the horizontal plane from true north. Right ascension is measured eastward along the equator from Aries. Declination is angular distance north/south of the celestial equator. Amplitude refers to bearing at rise/set on the horizon. Common Pitfalls: Mixing horizontal and equatorial systems; forgetting that hour angle increases to the west. Final Answer: Hour angle

Question 20

Astronomy for surveyors: Which statement correctly describes Polaris (the Pole Star) and the constellation Ursa Minor for practical field identification?

Accepted Answer

Introduction / Context: Surveyors often use the Pole Star to determine true north and to perform astronomical checks. Knowing the correct identity and location of Polaris relative to the constellation Ursa Minor (the Little Dipper) is essential for accurate field work at night. Given Data / Assumptions: The northern sky is considered, where Polaris is visible. “Pole Star” in current usage refers to Polaris, not any other star. Constellation names refer to patterns containing many stars, not single stars. Concept / Approach: Polaris is a single star positioned close to the north celestial pole. It is part of the constellation Ursa Minor and marks the end of the Little Dipper’s handle. Because it lies near the pole, the apparent diurnal motion is a small circle, helping surveyors align instruments to true north. Step-by-Step Solution: Identify the correct relationship: the Pole Star = Polaris (a single star).Identify its constellation: Polaris belongs to Ursa Minor.Therefore, the statement that correctly connects these facts is: “Polaris is the star commonly called the Pole Star and it lies in the constellation Ursa Minor.” Verification / Alternative check: Star charts show Polaris (Alpha Ursae Minoris) at the end of Ursa Minor’s handle, very close to the north celestial pole. Why Other Options Are Wrong: Ursa Minor always north of Polaris / Polaris always north of Ursa Minor: directions vary around the pole; these statements are not meaningful.“Pole Star” and “Polaris” different: incorrect; they are the same star in modern usage.Calling Polaris or Ursa Minor the name of the entire constellation: incorrect terminology. Common Pitfalls: Confusing the single star Polaris with the whole constellation or assuming “north of” has a fixed meaning around the celestial pole. Final Answer: Polaris is the star commonly called the Pole Star and it lies in the constellation Ursa Minor.

Advanced Surveying Questions