Skies Over Louisville: April 2009

Saturn (Hubble Heritage)

With the last vestiges of winter fading away and the spring days growing longer, April provides comfortable nights for skywatching. This month promises to deliver a virtual smorgasboard of constellations across the sky. The Full Moon occurs on April 9th, which means the Christian holiday, Easter, signifying rebirth and renewal, will occur on the following Sunday.

The planet Saturn steals the spotlight this month as the only planet visible in the evening and the brightest object in the night sky, save for the Moon. Saturn is located between the constellations Virgo and Leo. In fact, if you draw a line between the two brightest stars in these two constellations, Spica and Regulus respectively, this line will pass very close to Saturn. With a large collection of 500 to 1000 rings that span about 100,000 miles, these pieces of rock and ice make Saturn appear much brighter here on Earth. However, two times during Saturn's 29-year orbit around the Sun, these rings appear edge-on from Earth. Naturally, when we see these reflective rings edge-on, they appear to disappear and add nothing to Saturn's brightness, making and the planet appear unusually dim. The rings will be edge-on in September 2009, for the first time in almost 15 years.

The winter constellations are slowly sinking towards the western horizon and can only be seen in the early evening. Orion is marked in the sky by its characteristic three-star belt, which lies between the two bright stars, Betelgeuse, meaning "armpit", and the bright blue star Rigel. Below the belt of Orion, in his sword, is the Great Orion Nebula. Through binoculars this fuzzy patch is clearly visible as the gas and dust cloud that contains a stellar nursery, birthing a litter of young stars. By extending a line from the three stars in Orion's belt to the left you will encounter the brightest star in the sky, Sirius, the Dog star, in the constellation Canis Major. As Orion's trusty hunting dog, he is also accompanied by Canis Minor, or the little dog. By drawing a line from Rigel to Betelgeuse and continuing upward you will find a pair of bright stars. These are Castor and Pollux, the two brightest stars in the constellation Gemini.

The spring constellations become increasingly visible this month as the Big Dipper, a pattern of seven bright stars in Ursa Major, the Great Bear, rotates to its highest point in the sky. The Big dipper is our tour guide to the spring sky. A line drawn between the two leading stars in the bowl of the dipper will point to Polaris, the North Star. Polaris is located in the sky directly over the North Pole on Earth, so as the Earth spins Polaris appears to stay in the same spot in the northern sky. Polaris is also the brightest star in the constellation Ursa Minor, the Little Bear.

The Big Dipper can also point out stars in the southern sky. Follow the curved path of the handle and “arc to Arcturus”, the brightest star in the constellation Bootes, the Herdsman. His task is to shepherd the two bear constellations, Ursa Major and Minor, in their paths around the north star. If you continue along the curved line from the handle through Arcturus, it will lead to Spica, the brightest star in the constellation Virgo. The appearance of Virgo in the sky was said to bring about the end of the winter and the rebirth of Spring. A line drawn through the two toward the southern sky from the two back stars in the bowl of the Big Dipper will lead to Regulus, the brightest star in Leo, the Lion. This constellation is said to represent the Nimean Lion, a mythological beast slayed by Hercules as the first of his Twelve Trials.

To see the sky come to life and hear more stories of the seasonal sky, join us at the Rauch Planetarium for Skies Over Louisville: The Seasonal Edition on the first Saturday of every month at 9 p.m.

To Ask the Astronomer a question, send your email to planet @ louisville.edu.
To listen to the Sky hotline, call 502-852-6664 and press 4 for the Sky this Month.