People are complaining about Mercury in retrograde. But what does it actually mean?

People are complaining about Mercury in retrograde. But what does it actually mean?

Subject : Science and Technology

Section: Space technology

Context:

Humans found out retrograde motion was an optical illusion 500 years ago. However, the pseudoscientific practice of astrology continues to ascribe a deeper meaning to this illusion

Concept:

When we say that a planet is in retrograde it means that from the perspective of Earth, a planet’s motion across the sky goes backwards night after night compared to its usual direction for a period of time.

This happens when Earth overtakes a slower-moving planet in its orbit around the Sun, or is overtaken by a faster-moving planet.

Retrograde motion is technically called “apparent retrograde motion” since it’s not an actual change in the planet’s motion through space, but rather an effect caused by our perspective from Earth.

What causes Retrograde Motion?

Every planet moves around the Sun in the same direction. Normally, when you observe a planet’s location in the sky over several nights, each night it will appear to have moved a little farther in one direction relative to the stars behind it.

When a planet is in retrograde, it appears to move a little farther to the opposite direction each night instead.

This happens because the closer a planet is to the Sun, the faster it moves in its orbit around the Sun. This means that Mercury and Venus move through space more quickly than Earth, and will occasionally lap Earth just like runners moving more quickly around a track.

Likewise, the more distant planets Mars, Jupiter, Saturn, Uranus, and Neptune all move slower than Earth, and will occasionally be lapped by Earth.

Just like runners on a track, when a faster runner catches up to and then overtakes a slower one, the slower one’s motion relative to the faster one is backwards, even though both are moving forwards.

What does it mean when we say Mercury is “in retrograde”?

Retrograde motion works a little differently for Venus and Mercury. Because these planets orbit between Earth and the Sun, we tend not to see them high in the night sky. We mostly see them in the morning or evening, when they aren’t either directly between us and the Sun or on the far side of the Sun.

Most of the time, both planets move from east to west in Earth’s skies. But as Mercury, for example, speeds past Earth in its super-fast orbit and starts to turn around the Sun to pass to the other side, we see it slow in its east-to-west motion and start to move in the opposite direction as it takes that turn.

Because Mercury only takes 88 days to orbit the Sun compared to Earth’s 365, it overtakes us in this way three to four times a year, appearing to move backwards for about three weeks at a time. This frequency is probably the reason that Mercury’s is the most famous of the retrogrades