If Mars is in your mental rearview mirror following its close approach in late August, you might want to glance out your front window on the way home tonight. The red planet is set for another center stage appearance, this time in a celestial tango with the Moon.
The two objects will be near one another in the sky tonight and again Tuesday. They will appear closest just before dawn Tuesday.
The Moon is about 238,900 miles (384,402 kilometers) from Earth. Mars is now 35.7 million miles (57.5 million kilometers) away.
They can sometimes bunch up in the sky because both travel the same apparent path as viewed from Earth. This east-to-west arc, called the ecliptic, is also used by the Sun. It represents, in fact, the imaginary plane of the solar system in which nearly all the planets orbit the central star.
So the overnight event is a good time to examine celestial mechanics at work. It's also a great time to step out and marvel at two of the closest and most easily observed heavenly objects.
]Mars is slowly receding from Earth after a history-making pass on Aug. 27, when it was less than 34.65 million miles (55.76 million kilometers) away.
On that date, Mars shone at maximum brightness, magnitude -2.9 on a scale in which lower numbers represent brighter objects. Tonight Mars is still at magnitude -2.72. Through September, it continues to outshine all stars.
To find the pair, simply look to the southeast, where they will rise Monday night around 7 p.m. for those at mid-northern latitudes. Both the Moon and Mars are bright enough to be seen before night's darkness fully sets in. They'll be high in the south at midnight and then sinking into the southwest horizon near dawn. The scenario Tuesday night is similar, but they rise around 7:15.
If your view of the horizon is obstructed, you may not see them until later.
The proximity of the Moon and Mars changes slightly throughout the night. Astronomers measure the separation in degrees. Your fist on an outstretched arm covers about 10 degrees of sky.
In North America, westerners will get the best show. The two objects about 5 degrees apart Monday evening and then just 1 degree apart as dawn breaks Tuesday morning, according to Joe Rao, SPACE.com's Night Sky columnist.
From the eastern U.S., the Moon will hover about 6 degrees above and to the right of Mars as they rise Monday evening, Rao said.
"Since Mars is still extremely bright, people might want to watch for the rising of the Moon just before sunset and see if they can use it to locate Mars while it is still daylight," Rao suggested. A person's first daytime planet sighting can be an unforgettable experience.
By 5 a.m. Tuesday they'll be less than 3 degrees apart for eastern viewers, with the Moon below and to the right of Mars.
People in Siberia have a chance to see something even more remarkable. From some vantagepoints there, the Moon will actually pass in front of Mars, occulting it.
A similar close brush between occurred in July, and Mars was occulted by the Moon from parts of Florida. Several amateur astronomers
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