Now that Curiosity's safe and secure on the Red Planet and snapping photos of everything in sight, I hope it focuses its cameras on Earth sometime soon.
The rover sits inside the 96-mile-wide Gale Crater in Mars' eastern hemisphere just 5.4 degrees south of the equator. I was curious what the sky looks from Curiosity's location and fired up Stellarium to see.
It didn't take long to find Earth, low in the northeastern sky in morning twilight not far from the planet Venus.
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Since Earth is an "inner planet" from Mars' perspective, the same way Venus and Mercury are inner planets for us, it never strays too far from the sun and goes through phases just like the moon and other inner planets. Curiosity will see Earth best during morning and evening twilight. At Mars current distance from Earth of 154 million miles, our planet shines at magnitude -1.4 or nearly same as Sirius, the brightest star in the sky.
To the eye, Earth would shine a pale ocean water blue. Venus would still be the brightest planet (magnitude -3.0) but distinctly dimmer than when viewed from Earth, because it's farther from Mars than it is from our planet.
According to my calculation, the moon would be slightly less than one arc minute from Earth and probably not visible as a separate point of light with the naked eye. However, you could easily see it directly below Earth through a pair of binoculars. The two would appear as a beautiful double planet!
The moon is much darker than Earth and would only shine at magnitude 2.5, about the same brightness as one of the Big Dipper stars. Through a small telescope magnifying around 60x Earth would appear as a tiny gibbous moon or a little more than 3/4 full.
More sky wonders await Curiosity's cameras. Mars' two moons cycle through the sky just like our moon.
Phobos , the larger, is 14 miles wide and orbits only 3,700 miles from Mars' surface. It's so close that it moves around Mars faster than the planet rotates. Instead of rising in the east and setting in the west, Phobos rises in the west and sets in the east. Nuts, right? It moves so fast it crosses the entire sky in just four hours and 15 minutes. If you could be there in person, you'd see it move in real time like a very slow satellite. While Phobos is one of the darkest, least reflective bodies in the solar system, its proximity to the planet means it's brighter than you'd expect, easily outshining Earth and Venus at magnitude -5 at midmonth. Wait a minute - that's brighter than Venus is from Earth!
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The smaller moon Deimos is about 7 miles wide and orbits far enough from the planet to rise in the east and set in the west like our moon does. Things really get fun later this month on the morning of 31st. That's when Earth, Venus, Phobos and Deimos are all together in the eastern sky before sunrise. Wouldn't it be cool if NASA pointed one of the high-resolution cameras for an awesome family portrait?
One last tidbit. Mars' axis is tipped 25.2 degrees, nearly the same as Earth's 23.5 degrees. That's why both planet's have seasons. Despite similar inclinations, Mars' axis points to a different direction in the Martian sky. Earth's north polar axis points to the venerable North Star in the Little Dipper. Mars' "north star" is close to Deneb, the bright star that marks the head of the Northern Cross or constellation Cygnus. Mars' southern polestar is near the naked eye star Kappa Velorum.
It's fun and fascinating to imagine how the planets and stars look on other worlds, especially the one we're exploring with robotic eyes at this very moment. Seeing Earth from far away allows us to put our planet in perspective - we're a point of light dancing among the stars just like all the other planets.
