thumb|upright=1.5|thumbtime=24|An animation to explain the (apparent) retrograde motion of Mars, using actual 2020 planet positions

Apparent retrograde motion is the apparent motion of a planet in a direction opposite to that of other bodies within its system, as observed from a particular vantage point. Direct motion or prograde motion is motion in the same direction as other bodies.

While the terms direct and prograde are equivalent in this context, the former is the traditional term in astronomy. The earliest recorded use of prograde was in the early 18th century, although the term is now less common.

Etymology and history

thumb|250px|Apparent retrograde motion of Mars in 2003 as seen from Earth

The term retrograde is from the Latin word – "backward-step", the affix meaning "backwards" and "step". Retrograde is most commonly an adjective used to describe the path of a planet as it travels through the night sky, with respect to the zodiac, stars, and other bodies of the celestial canopy. In this context, the term refers to planets, as they appear from Earth, stopping briefly and reversing direction at certain times, though in reality we now understand that they perpetually orbit in the same uniform direction.

Although planets can sometimes be mistaken for stars as one observes the night sky, the planets actually change position from night to night in relation to the stars. Retrograde (backward) and prograde (forward) are observed as though the stars revolve around the Earth. Ancient Greek astronomer Ptolemy in 150 AD believed that the Earth was the center of the Solar System and therefore used the terms retrograde and prograde to describe the movement of the planets in relation to the stars. Although it is known today that the planets revolve around the Sun, the same terms continue to be used in order to describe the movement of the planets in relation to the stars as they are observed from Earth. Like the Sun, the planets appear to rise in the East and set in the West. When a planet travels eastward in relation to the stars, it is called prograde. When the planet travels westward in relation to the stars (opposite path) it is called retrograde.

This apparent retrogradation puzzled ancient astronomers, and was one reason they named these bodies 'planets' in the first place: 'Planet' comes from the Greek word for 'wanderer'. In the geocentric model of the Solar System proposed by Apollonius in the third century BCE, retrograde motion was explained by having the planets travel in deferents and epicycles.<gallery mode="packed" heights="200">

File:Retrograde Motion.bjb.svg|As Earth (blue) passes a superior planet such as Mars (red), the superior planet will temporarily appear to reverse its motion across the sky.

File:The astronomical explanation for Mercury retrograde.webm|An animation explaining why the planet Mercury may appear to move "backwards", or retrograde across Earth's sky

</gallery>Inner planets Venus and Mercury appear to move in retrograde in a similar mechanism, but as they can never be in opposition to the Sun as seen from Earth, their retrograde cycles are tied to their inferior conjunctions with the Sun. They are unobservable in the Sun's glare and in their "new" phase, with mostly their dark sides toward Earth; they occur in the transition from evening star to morning star.

The more distant planets retrograde more frequently, as they do not move as much in their orbits while Earth completes an orbit itself. The retrograde motion of a hypothetical extremely distant (and nearly non-moving) planet would take place during a half-year, with the planet's apparent yearly motion being reduced to a parallax ellipse.

The center of the retrograde motion occurs at the planet's opposition which is when the planet is exactly opposite the Sun. This position is halfway, or 6 months, around the ecliptic from the Sun. The planet's height in the sky is opposite that of the Sun's height. The planet is at its highest at the winter solstice, and at its lowest at the summer solstice, on those (rare) occasions when it passes through the center of its retrograde motion near a solstice. Note particularly that the hemisphere the observer is in is critical to what they observe. The December Solstice will place the planet high in the northern hemisphere sky where it is winter and place it low in the southern hemisphere sky where it is summer. The opposite is true if this happens at the June Solstice.

Since the planet's opposition retrograde motion is when the Earth passes closest, the planet appears at its brightest for the year.

The period between the center of such retrogradations is the synodic period of the planet.

{| class="wikitable"

|+ Planetary retrograde constants

|-

! Planet

! Synodic period (days)

! Synodic period (mean months)

! Days in retrogradation

|-

! Mercury

| 116

| 3.8

| ≈ 22 (19.85-24.19)

|-

! Venus

| 584

| 19.2

| 41

|-

! Mars

| 780

| 25.6

| 72

|-

! Jupiter

| 399

| 13.1

| 121

|-

! Saturn

| 378

| 12.4

| 138

|-

! Uranus

| 370

| 12.15

| 151

|-

! Neptune

| 367

| 12.07

| 158

|-

! Hypothetical far-out planet

| 365.25

| 12

| 182.625

|}

From Mercury

From any point on the daytime surface of Mercury when the planet is near perihelion (closest approach to the Sun), the Sun undergoes apparent retrograde motion. This occurs because, from approximately four Earth days before perihelion until approximately four Earth days after it, Mercury's angular orbital speed exceeds its angular rotational velocity. Mercury's elliptical orbit is farther from circular than that of any other planet in the Solar System, resulting in a substantially higher orbital speed near perihelion. As a result, at specific points on Mercury's surface an observer would be able to see the Sun rise part way, then reverse and set before rising again, all within the same Mercurian day.

See also

  • Deferent and epicycle
  • Retrograde and prograde motion
  • Hipparchus
  • Ptolemy
  • Shen Kuo
  • Spherical astronomy
  • Wei Pu

References

  • Animated explanation of the mechanics of a retrograde orbit of a planet , University of South Wales
  • NASA: Mars retrograde motion
  • Double sunrises, 3DS Max Animation – illustrating the case of Mercury (the animation of an imaginary apparent retrograde motion of the Sun as seen from Earth begins at 1:35)
  • Mars Looping – The Retrograde Motion of Mars – 2018