SkyEye

Retrograde renders me wrinkled and wrecked
No one moves forward and nothing’s correct
A period of panic
Malfunctions and static
Till the planets and orbits have all gone direct
— Fred Hornaday, "Mercury Mayhem", 2020

Apparent Retrograde Motion

Retrograde motion can mean several different things to astronomers. It might refer to the direction in which a planet spins on its axis or the direction in which an object orbits another. Possibly the most familiar use of the term relates to the apparent motion of a planet amongst the background stars.

The planets normally move from west to east against the background stars. This is termed direct or prograde motion. However, occasionally a planet's track across the celestial sphere slows, stops and then reverses direction for a period of time. This east-to-west motion is known as apparent retrograde motion or simply retrograde. Of course, the planet is not really changing directions in space. This change in direction is an optical illusion caused by the differences in orbital velocities of the Earth and the other planet.

Retrograde motion was a major headache for ancient astronomers. In the geocentric model where Earth was at the centre of the universe and everything — the Moon, the Sun, the planets, the stars — moved around it in circular orbits. Retrograde motion simply could not exist in such a universe. However, the ancient astronomers were nothing if not inventive and they devised a complicated system involving epicycles (circles within circles) to explain the occasional backward motion of the planets. This explanation was so successful that it held sway for centuries. In reality, the truth was much simpler and the heliocentric model of Copernicus, with the Sun at the centre of the solar system, showed the way.

The accompanying diagram illustrates a case of retrograde motion exhibited by a superior planet such as Mars. Earth is closer to the Sun than any superior planet and so it orbits the Sun more quickly. Thus, from time to time Earth overtakes the superior planet. It is around this time that the superior planet goes into retrograde.

1. Normal direct motion as Earth begins to overtake the superior planet.
2. Decelerating direct motion as Earth overtakes the superior planet.
3. The superior planet reaches a stationary point. Retrograde motion commences.
4. Opposition: Earth finally catches up with the superior planet.
5. The superior planet reaches a stationary point. Direct motion commences.
6. Accelerating direct motion as Earth begins to leave behind the superior planet.
7. Normal direct motion as Earth leaves behind the superior planet.

The path a planet makes against the background stars may be S-shaped (as in the diagram) or it may make a loop. This latter case is often referred to as a retrograde loop. The shape of the retrograde motion depends on the position of the planet in its orbit. If the planet is near a node (the place in a planet's orbit where it crosses the ecliptic), then the retrograde motion is S-shaped. However, if the planet is near its maximum declination north or south, a loop is formed. The higher the orbital inclination (angle to the ecliptic), the more open the loop or S-shape.

Inferior planets also exhibit retrograde motion. Whereas superior planets undergo retrograde motion centred around the time of opposition, inferior planets go into retrograde before inferior conjunction.

The diagram below shows the annual track of the inferior planet Mercury during 2019. It enters retrograde motion three times during the year and demonstrates both kinds of retrograde behaviour. The first two instances are loops but in November, Mercury is at its ascending node during inferior conjunction and thus, the retrograde motion is S-shaped.

A planet enters into retrograde motion once every synodic period. For instance, Mercury goes into retrograde for about three weeks every four months. Venus, on the other hand, spends six weeks in retrograde but this only happens every 19 months. Mars is even more extreme, with retrograde periods occuring every two years and lasting two months or more. The further out from the Sun you go, the shorter the period between retrogradations but the longer a planet spends in retrograde. Neptune spends nearly half of the year in retrograde!

Other Forms of Retrograde Motion

As was mentioned above, there are other types of retrograde motion in the solar system. The eight planets orbit the Sun in direct motion. This means that they are orbiting in the same direction that the Sun rotates on its axis. This makes sense as it is thought that the solar system formed out of a spinning disk of gas. However, not all objects orbit the Sun in this manner. There are a few asteroids and a number of comets (including Comet 1P/Halley) that orbit in the Sun in the opposite direction to the Sun's spin. These objects are in what are known as retrograde orbits.

Most of the large moons in the solar system also orbit in direct or prograde motion. Notable exceptions are Triton, the largest satellite of Neptune, and Phoebe, the outermost large satellite of Saturn. A number of smaller satellites orbit their primaries in retrograde fashion. It is thought that these objects are captured asteroids.

A planet with an axial tilt of over 90° is described as having retrograde rotation. There are two examples in the solar system. The first is Venus. With an axial tilt of 177°, it rotates backwards with respect to its orbital direction. Uranus has an axial tilt of 98°. Technically it also has retrograde rotation, appearing to roll 'on its side'.