In astronomy, one astronomical unit (1 AU) is defined as the average distance between which two celestial bodies?

Introduction / Context:
Astronomical distances are very large, so it is convenient to use special units rather than everyday units like kilometres. One common unit is the astronomical unit, abbreviated as AU. This unit is defined based on the average distance between two familiar bodies in the Solar System. The question checks whether you know which pair of bodies is used to define 1 AU.

Given Data / Assumptions:

• We want the definition of one astronomical unit.
• It is based on the average separation of a planet and the Sun.
• The unit is widely used in describing distances inside the Solar System.
• We neglect details of orbital eccentricity and focus on the definition only.

Concept / Approach:
By convention, astronomers define one astronomical unit as the average distance between Earth and the Sun over one complete orbit. This distance is roughly equal to 1.5 times 10 to the power 8 kilometres and serves as a convenient yardstick. Distances of other planets from the Sun or between planets are often expressed in multiples of AU. For example, Jupiter orbits at about 5.2 AU from the Sun. Therefore, among the options, the correct pair is Earth and the Sun.

Step-by-Step Solution:
Step 1: Recall that AU is a unit chosen to relate directly to Earth orbit, because Earth is our reference planet. Step 2: Remember that 1 AU represents the average distance between Earth and the Sun, not between Earth and its Moon. Step 3: Understand that other planet Sun distances are measured relative to this standard unit. Step 4: Check the options and locate Earth and the Sun as option A. Step 5: Select Earth and the Sun as the correct answer for the definition of one astronomical unit.

Verification / Alternative check:
Common astronomy references and textbooks state that 1 AU is about 149.6 million kilometres, the mean distance from Earth to the Sun. The distance between Earth and the Moon is about 384 thousand kilometres, far smaller and not used as the AU definition. Distances for Jupiter, Mars or Pluto are several AU and not used as the base unit. This confirms that Earth Sun distance is the correct defining pair for 1 AU.

Why Other Options Are Wrong:
Earth and the Moon correspond to a much smaller distance and are not used as the basis of the AU.
Jupiter and the Sun are separated by several AU and that distance varies; it is expressed using AU rather than defining AU.
Pluto and the Sun likewise are many AU apart and do not define the unit.
Mars and the Sun form a pair whose separation is about 1.5 AU, not the definition of 1 AU.

Common Pitfalls:
Some learners confuse 1 AU with any planet Sun distance and think it may vary for each planet. Others mix up Earth Moon distance with Earth Sun distance, especially if they do not remember the approximate numerical values. To avoid errors, remember that AU is anchored to the Earth Sun system, making it a natural standard for measuring relative positions of planets in the Solar System.

Final Answer:
One astronomical unit is defined as the average distance between Earth and the Sun.