Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2016 March 3

Moons and Jupiter
Image Credit & Copyright: Phillip A Cruden

 

Explanation: Some of the Solar System’s largest moons rose together on February 23. On that night, a twilight pairing of a waning gibbous Moon and Jupiter was captured in this sharp telescopic field of view. The composite of short and long exposures reveals the familiar face of our fair planet’s own large natural satellite, along with a line up of the ruling gas giant’s four Galilean moons. Left to right, the tiny pinpricks of light are Callisto, Io, Ganymede, [Jupiter], and Europa. Closer and brighter, our own natural satellite appears to loom large. But Callisto, Io, and Ganymede are actually larger than Earth’s Moon, while water world Europa is only slightly smaller. In fact, of the Solar System’s six largest planetary satellites, only Saturn’s moon Titan is missing from the scene.

Use Big Dipper to find Mizar and Alcor

Tonight, find the Big Dipper – known as the Plough in the U.K. or, in Hindu astronomy Saptarishi, after the seven rishis. The chart at top shows the Big Dipper on an evening in March, ascending in the northeast during the evening hours. If you have a dark sky – and good vision – you might be able to see the faint star Alcor next to Mizar, the middle star in the Big Dipper’s Handle.

Still not sure the pattern you’ve found is the Dipper? Try to notice if the pattern you see matches the chart shown at right. That chart shows a well-known trick for finding the North Star, or Polaris. That is, the two outermost stars in the bowl of the Big Dipper point to Polaris. Those stars are Dubhe and Merak. They are well known among amateur astronomers as The Pointers. If you extend a line about five times the distance between Dubhe and Merak from the star Dubhe – and don’t find a medium-bright star (Polaris) – then the pattern you see is not the Big Dipper. Keep looking.

An ancient eye test for those wishing to join the Roman army involved spotting stars in the Handle of our modern-day Big Dipper. You can take this ancient eye test, too.

Go outside around 9 p.m. You should see the Big Dipper just off the northeast horizon. The middle star in the handle of the Big Dipper is Mizar. If you look for a couple seconds longer, you may see a little starry point right next to Mizar.

This second star is called Alcor. If you had lived in the time of the early Romans and could see Alcor, you would have been eligible to be an archer in the Roman army. If not, you likely would have served in another capacity for the Caesar. It’s said that sultans of the past also tested their soldiers’ eyesight in this way.
Bottom line: As darkness falls, look for the Big Dipper and the star Mizar in the northeast sky. With either the unaided eye or binoculars, seek for Mizar’s nearby companion: Alcor.Mizar and Alcor form a “visual double” star. At one time, astronomers doubted that Mizar and Alcor were gravitationally bound to one another. But Mizar is now thought to be four stars in one, and Alcor two stars in one. Given that Mizar and Alcor likely revolve around each other, that makes for six stars in an intricate gravitational dance.

 

EarthSky