One of Saturn’s moons hides a star

¡SkyCaramba! Weekly astronomy blog for the week ending September 13, 2014

Rhea, a moon of Saturn, photographed by the Cassini spacecraftOne of Saturn’s moons is about to hide a star in Libra. Saturn is a naked eye object. Its moon Rhea and the star Rhea will cover on the evening of September 12th are not. But a small telescope, the right location, and clear enough weather will enable you to see the star “wink out”.

Rhea is the ringed planet’s second largest moon. It’s made mostly of water ice and there’s a little rock in the mix too. Even though the moon orbits Saturn, it’s named for a daughter of the sky god Uranus. Giovanni Cassini is the first person known to see Rhea in late 1672.

Rhea is marked with lots of craters. It has a very thin atmosphere of mostly oxygen with some carbon dioxide. And it orbits Saturn in just four-and-a-half days.

The star Rhea will hide for almost a full minute is a white star about 306 light years away. It shines at seventh magnitude, about three magnitudes dimmer than you should expect Rhea to appear. A dark sky and clear enough optics will help you see it. The star is known in the catalogs as SAO 159034 and HIP 74007.

People in the eastern third of the United States and eastern third of Canada will see the occultation by looking to the southwest on the evening of September 12 local time. The exact time the star disappears will vary by up to about a minute. In the Universal Time zone, it will be at about 00:39 on the 13th.

Rhea star occultation visibility mapEven if you’re in a good spot and even if the weather looks clear, atmospheric turbulence could trouble your view. Saturn is kind of low in the sky just after sunset. The star may appear to dim and brighten quickly and repeatedly just because of the extra twinkling we’ve come to expect when looking at low objects.

And the farther east you are, the better. The occultation happens not long after sunset as you go west. Those seventh magnitude stars don’t come out until it’s pretty dark. In most of Illinois, the sun will have been below the horizon less than a half hour when the occultation happens.

Many of the astronomers who look at occultations just like to see interesting things. But serious scientists take an interest too. The way SAO 159034 goes dim and for how long can help them figure out how big the star is and whether there’s another star orbiting it very closely. If we didn’t already know so much about Rhea, they would also be able to determine its size, its shape, whether it has an atmosphere, and whether other objects are traveling with it.

Look around when you’re out. Just like on the night Cassini discovered Rhea, Saturn will be a little west of Mars in the sky. But Cassini also had a crescent moon and Venus to look at that December evening.