Tonight – November 25, 2014 – the waxing crescent moon and the red planet Mars appear in the southwest sky at nightfall. Be sure to check out these worlds at early evening because they’ll follow the sun beneath the horizon by mid-evening.
While our Earth has one large ball-shaped moon, Mars has two tiny potato-shaped moons. Our moon lies at a mean distance of 384,400 kilometers (238,855 miles), but Mars’ two moons reside very close to the surface of the red planet. Deimos, the smaller of Mars’ two moons, is 23,460 kilometers (14,577 miles) from Mars.
This illustration compares the relative sizes of Mars’ moons as seen from the surface of Mars, with the size of our moon as seen from Earth. Image via NASA/JPL-Caltech/Malin Space Science Systems/Texas A&M Univ.
Phobos, the larger, lies only 9,270 kilometers (5,670 miles) away. These distances are from the center of Mars. These moons lodge closer yet to Mars’ surface.
The closer a moon’s orbit, the faster the moon goes in its orbit around its parent planet. Deimos’ orbital period is 30 hours and 12 minutes. But the orbital period of Phobos, the closer moon, is only 7 hours and 39 minutes. Phobos is one of the very few moons in the solar system to orbit a planet in less time than its parent planet’s rotational period. Mars rotates full circle in 24 hours and 39 minutes, so one day on Mars is only slightly longer than one Earth day.
The synchronous orbit of two artificial satellites. In this image, we’re looking downward at the Earth’s north pole, and the satellites are orbiting above the Earth’s equator. At a distance of 35,786 kilometers above the Earth’s surface (42,164 kilometers from the Earth’s center), the satellites’ orbital periods equal Earth’s rotational period. Image via Wikipedia
The distance at which a moon’s (or an artificial satellite’s) orbital period equals the planet’s rotational period is called the synchronous orbit radius. On Mars, this distance is 17,031 kilometers (10,583 miles) above Mars’ surface, or 20,427 kilometers (12,693 miles) from the center of Mars. Farther away than the synchronous orbit, the moon’s orbital period is longer than the planet’s rotational period. Below the synchronous orbit, the moon’s orbital period is shorter than the planet’s rotation.
Because Phobos circles Mars below the synchronous orbit distance, the orbit of this moon is decaying, with Phobos moving closer to Mars at the rate of about 1.8 meters (6 feet) per century. Astronomers believe Phobos is doomed to crash into Mars or to break up into a ring of rubble in about 50 million years.
At a distance of 60 Earth-radii (384,400 kilometers), our moon resides at roughly 9 times the Earth’s synchronous orbit radius of 6.6 Earth-radii (42,000 kilometers from the Earth’s center). At present, the moon is retreating from Earth at the rate of about 3.8 meters (12.5 feet) per century.
Bottom line: On the evening of November 25, 2014, as darkness falls, watch for the picturesque pairing of the waxing crescent moon with the red planet Mars.