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. Show 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. From Earth : When standing on the Earth looking up at the sky, it would appear that the Moon travels from east to west, just as the Sun and the stars do. Day after day however, the Moon appears to move to the east with respect to the stars. In actual fact, the Moon orbits the Earth from west to east, as do the vast majority of manmade satellites such as the International Space Station. The apparent westward motion of the Moon from the Earth’s surface is actually an artifact of it being in a supersynchronous orbit. This means that the Earth completes one sidereal rotation before the Moon is able to complete one orbit. As a result, it looks like the Moon is travelling in the opposite direction, otherwise known as apparent retrograde motion. This phenomenon also occurs on Mars, which has two natural satellites, Phobos and Deimos. Both moons orbit Mars in an eastward (prograde) direction; however, Deimos has an orbital period of 1.23 Martian sidereal days, making it supersynchronous, whereas Phobos has an orbital period of 0.31 Martian sidereal days, making it subsynchronous. Consequently, although both moons are traveling in an eastward (prograde) direction, they appear to be traveling in opposite directions when viewed from the surface of Mars due to their orbital periods in relation to the rotational period of the planet. Apparent path of Mars in 2009–2010 relative to the constellation Cancer, showing its “opposition loop” or “retrograde loop” The asteroid 514107 Kaʻepaokaʻawela has a retrograde orbit. Its apparent retrograde motion occurs at superior conjunction with the sun, as shown in this example in 2018. All other planetary bodies in the Solar System also appear to periodically switch direction as they cross Earth’s sky. Though all stars and planets appear to move from east to west on a nightly basis in response to the rotation of Earth, the outer planets generally drift slowly eastward relative to the stars. Asteroids and Kuiper Belt objects (including Pluto) exhibit apparent retrogradation. This motion is normal for the planets, and so is considered direct motion. However, since Earth completes its orbit in a shorter period of time than the planets outside its orbit, it periodically overtakes them, like a faster car on a multi-lane highway. When this occurs, the planet being passed will first appear to stop its eastward drift, and then drift back toward the west. Then, as Earth swings past the planet in its orbit, it appears to resume its normal motion west to east.[4] 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 center of the retrograde motion occurs when the body is exactly opposite the sun, and therefore high in the ecliptic at local midnight. The retrogradation 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 period between the center of such retrogradations is the synodic period of the planet. When planets in our Solar System go into retrograde, you’ll often hear them being blamed as the source of a person’s woes, restlessness, burnout or, more optimistically, representing a time for reflection. In reality, however, there is very little scientific evidence that any of the planets going into retrograde will have a tangible effect on our lives. Whether or not you believe this, planets going into retrograde is a common and regular occurrence, and in September 2022, there will be three planets in retrograde at the same time… four if you include Pluto. But what exactly does retrograde mean? What planets are in retrograde right now? And what causes the apparent retrograde motion of the planets? Answers to these questions, and more, are below. If you’re looking forward to breathing in the night air this year, we’ve put together this full Moon UK calendar and astronomy for beginners guide to help you plan ahead. There are also regular meteor showers every year and we’ve rounded them all up in this handy meteor shower calendar. What is retrograde?The planets move from west to east across the night sky. All the planets travel in the same direction around the Sun, and if you imagine a spot above the Earth’s north pole, they would be seen to travel anticlockwise. Anticlockwise motion is the ‘normal’ (more common) motion and is known as ‘prograde’. When a planet is going through a period of apparent retrograde motion, it appears as though it’s moving in the opposite direction, from east to west across the sky, often looping or zig-zagging as it goes. To the observer, this looks as though the planet is moving backwards. The word ‘retrograde’ itself, means to move backwards. If a planet suddenly started moving backwards, it’s perhaps not surprising that our ancestors might have taken this as a sign of impending doom. What causes apparent retrograde motion?This backwards movement, retrograde, is actually an illusion created by our viewpoint here on Earth. It’s created by the different speeds at which the planets orbit the Sun. When the Earth’s orbit overtakes a planet with a slower orbit, we see that planet from a different perspective. As Earth overtakes another planet, let’s say Jupiter, it overtakes on the inside and from our viewpoint on Earth, this results in Jupiter appearing to zigzag across the sky. More like this Think of when you overtake a vehicle on the motorway; you catch up with the other car, then you’re level with each other, and then the other car appears to move backwards as you pull ahead. This is why you’ll often hear retrograde referred to as ‘apparent retrograde motion’ – as it only appears as though the planet is moving backwards, it’s not really. The superior planets – any of the planets whose orbit is further away from the Sun than the Earth’s – go into retrograde around the same time as they are in opposition. In astronomy, opposition is when a planet appears to be in the opposite direction to the Sun, as viewed from Earth. Oppositions occur approximately once a year for each planet (except Mars, which is around every 26 months), and occur as the Earth passes between that planet and the Sun. It’s also when the planet in question is closest to the Earth, making it appear bigger and brighter, so it’s an excellent time to observe these distant worlds. Because Mercury and Venus have orbits that are closer to the Sun than the Earth’s, they can never go into opposition. However, planets closer to the Sun can go into retrograde. When is the next retrograde in 2022?
Mercury in retrogradeTravelling the fastest out of all the planets, it only takes approximately 88 days for Mercury to orbit the Sun (and earning it the nickname the Swift Planet). This means it overtakes us three or four times every year, and therefore goes into retrograde three or four times a year, lasting around three weeks at a time. Mercury in retrograde is relatively common, but sometimes, it will overlap with a rather exciting astronomical event: a transit. A transit occurs when the Sun, Mercury and the Earth line up, so that from our point of view, Mercury appears to move across the Sun as a tiny black disk. Venus in retrogradeIt takes 225 days for Venus to orbit the Sun and goes into retrograde every 18 months, lasting around six weeks at a time. Mars in retrogradeMars goes intro retrograde approximately every 26 months, beginning around five weeks before opposition. Mars in retrograde lasts for a few weeks before the planet appears to move forward again. Its distinctive yellowy-orange colour makes the Mars retrograde one of the easiest to follow with the naked eye. The outer planetsThe outer planets, also called the Jovian planets, go into retrograde less often, but appear to be moving backwards for longer – getting progressively longer the further away from Earth:
Pluto in retrogradeWe all know Pluto should be reinstated as our ninth planet, and for those of a superstitious nature, look away: Pluto goes into retrograde for around half the year, every year. It’s the largest-known object in the Kuiper Belt (a doughnut-shaped region of icy bodies beyond Neptune) and last went into retrograde on 29 April 2022, lasting until 28 October 2022. In 2023, Pluto will go into retrograde on 1 May, remaining until 11 October. Retrograde and the Ptolemaic modelIn 150 AD, the Greek astronomer Ptolemy theorised that the Earth was at the centre of the Universe, with the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn on ‘spheres’ around the Earth, in that order. Uranus and Neptune were unknown at that time, having been discovered in 1781 and 1846 respectively. So, with the Earth at the centre, how did the Ptolemaic model explain the apparent retrograde motion of the planets? Put simply, Ptolemy explained this by proposing that the planets moved in small circles (called epicycles), which in turn circled the Earth on their respective spheres (cycles). The motion of the smaller epicycles neatly explained the apparent backwards movement exhibited by the bright planets. It wasn’t until 1610 that Galileo Galilei disproved the Ptolemaic model by discovering that, like the Moon, Venus went through phases; something incompatible with Ptolemy’s take on the Solar System. If Venus was positioned, at all times, between the Earth and the Sun, then the planet would always appear as a crescent or all dark – but Galileo observed distinct phases. Below is an animation explaining Ptolemy's model of the Universe, with Earth in the centre: Read more: |
Apparent retrograde motion occurs when a planet appears to
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