# How does the retrograde motion of Mercury compare with that of Mars?

Apr 7, 2016

When Mars is viewed from Earth, it shows retrograde motion in the sky when Earth goes past it. Mercury never shows this motion.

#### Explanation:

Because all planets orbit the Sun in the same direction, they normally appear to move in the same direction relative to most stars from day to day (or night to night) -- namely west to east. But when Earth passes directly between the Sun and a more distant planet like Mars, the combination of our planet's faster motion plus our relatively close distance to the other planet makes that planet temporarily appear to shift westwards.

If we go to Mercury, we will find Earth undergoing retrograde motion when we look back to it, every time Mercury laps Earth in its orbit.

Check out the pictures shown here:

Apr 7, 2016

Mercury turns about the Sun faster than the Earth and the Earth is faster than Mars. Earth tends towards Mercury and Mars tends to move towards the Earth. .

#### Explanation:

Retrograde motion is the apparent opposite motion of one moving body relating to another moving ahead.

The orbital periods of Mercury, the Earth andMmars are 88d, 365,26d and 787d, nearly.

The angular speeds about the Sun are nearly ${4}^{o} , {1}^{o} \mathmr{and} {0.5}^{o}$, per day..
So, Mercury turns around the Sun about 4 times as fast as the Earth and Mars turns at nearly half the angular speed of the Earth.

For another comparison of relative angular speeds:
In circling around the Sun, Earth and faster Mercury will be in the same heliocentric longitudinal plane, once in about 116 days, For the faster Earth and Mars, this period is more than 2 years.