The Planet Uranus Passes a Star

Posted by Brian Ventrudo

The next week or so gives you an excellent chance to easily spot the planet Uranus.   The 6th-magnitude planet appears to pass a star of nearly the same brightness, and the Great Square of Pegasus shows you the way.  Here’s how to find the 7th of our solar system planet with binoculars, telescope, or even without optical aid if you have dark sky…

The Great Square of Pegasus points the way to Uranus in late September 2012, as seen in the northern hemisphere. Click to enlarge.

To find Uranus over the next week, look east at about 9 p.m. local time for the Great Square of Pegasus, which is tilted right now and looks more like a big diamond (see map above). Find the easternmost stars in the square. These are Alpheratz and Algenib. They’re about 14º apart.  Now follow a line from Alpheratz through Algenib with your binoculars or finderscope.  First you come to the orange star 41 Piscium.  It shines at magnitude 5.3 and lies about 6.5º from Algenib.  Now keep going on the same line another 6º or 7º until you get to a closely-spaced pair of 6th-magnitude “stars”.  The easternmost star is the star 44 Piscium.  The westernmost is the planet Uranus (see below).

Uranus and the star 44 Piscium on Sept. 17, 2012.

If you have dark sky, try to split the pair with your unaided eye (wait till later in the the week, perhaps Sept. 25 or later).  And if you have a telescope, crank up the magnification to 100x or more.  Uranus will reveal a tiny disk while the star 44 Psc will remain point-like.  The planet’s disk will get larger as you increase magnification.  This is the same approach William Herschel used in 1781 to identify Uranus as a planet, the first planet discovered with a telescope.  Look also for a subtle colour difference between the two objects.  In a telescope, Uranus is pale aquamarine, while 44 Piscium will appear yellow-white.

The star and planet are just 2 arcminutes apart on September 22-23, then Uranus starts to slowly pull away.  You can follow the motion of Uranus relative to the star over the next few weeks.  Uranus doesn’t change position quickly… it takes about 84 years to make a single circuit of the sky.

UPDATE:  Here’s an image I took on September 24 at 03:00 UTC showing Uranus and 44 Piscium near their closest approach…