Saturn, Spica, and Mars

¡SkyCaramba weekly astronomy blog for the week ending June 23, 2012!

In two months, two planets and a bright star will line up in the evening sky. You can start watching them position themselves for this meeting. Two of these objects are actually in place right now.

In the northern hemisphere, look to the southwest in the evenings to find Saturn and Spica. Saturn will be a little north of Spica. You may recognize Leo to the northwest or Boötes to the northeast. Saturn has been somewhat close to Spica since last year. Spica is the brightest star in Virgo.

While Earth and Saturn orbit the sun, the ringed planet makes a curved back-and-forth motion in our sky. More of its motion is easterly than westerly. So over long periods, Saturn makes it way around the entire sky. Mars moves in a similar manner in our sky. But because it’s closer to the sun than Saturn, it orbits faster. Consequently, its back-and-forth motion brings it around the entire sky in less time.

The sidereal period of Mars is 1.88 years. Saturn’s is 29.65 years. Because Mars makes it around the entire sky in less than two years, wherever Saturn is, Mars will pass it in about that amount of time. The two planets were together last in July and August 2010. They’ll be together again in August 2014. But it usually takes more than 50 years for them to appear together again in the same section of the sky. Saturn and Mars were close to each other west of Spica in 1951 and east of it in 1953. After this year, the planets will be close to the star again in 2070.

In late June, we find the ringed planet north of Virgo’s bright star. Mars is just entering the constellation’s modern boundary after a long visit to Leo. Night to night, you’ll be able to see the red planet inching toward Saturn. It will be south of the star Porrima around July 22. It will be right between Saturn and Spica on August 14. Enjoy that view. They won’t put on another show together for 58 years. While you’re watching from night to night, compare Mars to the red-orange Arcturus in Boötes.

The other planets make their circuits around the sky in different amounts of time. Jupiter takes nearly 12 years. Uranus finishes the tour in 84 years. And Neptune makes it all the way around in 165 years. It has had barely one entire orbit since its discovery in 1846.

Because Mercury and Venus are closer to the sun than Earth, their tours of the sky vary wildly compared to those of the outer planets. While Mercury can pass by the same star in as little as two months, it could also take about a year. Venus may spend just a few months away from a particular point in the night sky. Then, it could be nearly a decade before it visits again. There are links below to explain further.