What is the aphelion of a celestial body?

The aphelion of a celestial body refers specifically to the point in its orbit where it is furthest from the Sun. In the context of our solar system, planets and other celestial bodies follow elliptical orbits as described by Kepler's laws of planetary motion. An elliptical orbit has two focal points, one of which is occupied by the Sun. As a result, there are points in the orbit where the celestial body is closer to the Sun (perihelion) and points where it is farther away (aphelion). This distinction is crucial for understanding the variations in distance between planets and the Sun throughout their orbits and can affect aspects such as the planet's orbital speed and the intensity of solar radiation it receives.

The other options mention concepts that do not pertain to the definition of aphelion. For instance, the closest point to the Earth in an orbit is referred to as perigee in the case of satellites, not aphelion. Retrograde motion describes a phenomenon where a planet appears to move backward in the sky from our perspective on Earth, which is unrelated to its distance from the Sun. Lastly, the highest point in the sky for a planet relates to its position in the sky as observed from Earth, rather than