by Dominic Ford, Editor

Mars at opposition in 2001, as seen by the Hubble Space Telescope

An apparition of an object is a period of weeks or months during which it is visible in the night sky.

All of the planets go through such periods when they are well-placed for observation, punctuated by other times when they pass too close to the Sun to be readily observed.

The outer planets

The apparitions of the outermost planets in the solar system – Saturn, Uranus and Neptune – last roughly 6-9 months, and repeat once a year.

These planets feel a much weaker gravitational pull towards the Sun than the Earth and orbit very slowly, taking decades to complete each circuit. This means they appear to move very slowly through the constellations, barely moving from one month to the next relative to background stars behind them.

By contrast, the Earth orbits the Sun once a year, making the Sun appear to make a complete circuit through the constellations over the same period. The Sun follows the zodiac and comes back to the same point among the constellations on the same date each year.

This motion is much faster than that of the outer planets, and as a result, the rising and setting times of these planets changes from one day to the next primarily because of the Sun's motion rather than their own.

They cycle between solar conjunction and opposition in a little over one year.

The inner planets

The inferior planetsMercury and Venus – are the ones that orbit closer to the Sun than the Earth.

They move across the sky in a very different way from the outer planets. The outer planets alternate between solar conjunction and opposition. At solar conjunction, the solar system is configured with the planet opposite to the Earth in its orbit, with the Sun in the middle. The planet appears to pass behind the Sun. At opposition, the planet once again forms a straight line with the Sun and Earth, but this time with the Earth in the middle, such that the planet appears opposite the Sun in the sky.

By contrast, the inner planets never venture very far from the Sun. Venus's maximum separation from the Sun is 48°, while Mercury's is 28°. They can never appear opposite the Sun in the sky. Instead, they pass between the Earth and Sun, appearing to pass in front of the Sun at inferior conjunction.

Between the times when the inner planets pass in front of or behind the Sun, they appear alternately in the morning and evening skies, depending whether they lie to the east of the Sun or to the west.

The Earth's neighbors

The apparitions of the planets closest to the Earth – Venus and Mars – occur least frequently. Because they orbit the Sun at similar distances to the Earth, they have similar orbital periods to it, and travel around their respective orbits with similar speeds to it.

This means that their motion relative to the Earth is small, and the Earth passes Venus and Mars in their respective orbits comparatively rarely.




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