Moon and Mars again on October
Tonight for October 28, 2014
Tonight … watch as the waxing crescent moon and the planet Mars light up the southwest at nightfall on October 28. As evening deepens, the celestial twosome sinks westward and follows the sun beneath the horizon by mid-evening.
Even so, the moon is really moving eastward in its orbit, in the same direction in which our planet Earth rotates, from west to east. Earth rotates once relative to the background stars in 23 hours and 56 minutes, whereas the moon takes 27.3 days to complete its orbit. Earth’s relatively fast rotation makes it appear as if the moon is literally traveling westward each day, rising in the east and setting in the west.
Let’s take a pretend trip to Mars, to view its two tiny moons – Deimos and Phobos – in the Martian sky. Like Earth, Mars rotates eastward upon its rotational axis. A day on Mars is only a tiny bit longer than on Earth, being some 24 hours and 40 minutes long. So the rotational periods of the Earth and Mars are virtually the same.
Yet, in contrast to our moon’s relatively long orbital period, these two Martian moons speed around Mars very quickly. The farther moon, Deimos, goes around Mars in about 30 hours, while the closer moon, Phobos, circles Mars in about 8 hours.
Deimos, like our moon, rises in the east and sets in the west, but it takes several days for Deimos to cross the Martian sky. Our moon, on the other hand, goes across Earth’s sky in about 12 hours, although this period of time can vary, depending on the season and the moon’s phase.
Because Phobos’ orbital period is way shorter than Mars’s rotation period, you’d actually see the moon Phobos rising in the west and setting in the east on the red planet. No known solar system moon orbits so closely to its parent planet, zooming around Mars three times in one day.
Bottom line: The waxing crescent moon and Mars can be found in the southwestern sky, starting at nightfall, on the evening of October 28, 2014.