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New things to see as spring/summer approaches

April 5, 2011
By Aileen O’Donoghue (aodonoghue@stlawu.edu)

The winter sky will disappear abruptly as the apparent westward motion of the stars meets the sun's northward motion lengthening the day. By the end of May, Orion will set before the sky darkens. So this is the time to explore this region with binoculars.

From the Orion Nebula, where the Hubble Space Telescope has imaged stars at all stages of formation, across the swath of Milky Way in which the twins of Gemini bathe their feet, to the Beehive and Coma star clusters in Cancer and Coma Berenices, this part of the sky is rich with glittering beauty.

As shown in Figure 1, the upside-down Y of Cancer, nestled between the bright constellations of Gemini and Leo, is easy to overlook with its few faint stars. However, in the heart of the crab is a cluster of young stars known as the Beehive Cluster and the Praesepe (PRAY'-suh-pee), Latin for "manger." On a dark night, it's fairly easily seen with the unaided eye as a fuzzy blur about midway between Pollux and Regulus. Binoculars reveal a few of its hundreds of stars. This sibling group, an "open" cluster, is about 580 light years away, 10 light years across and 400 million years old. Since it's visible to the naked eye, it has been known since ancient times. Its fuzziness led to it being called a manger and the stars nearest to it Ascellus (uh-SELL'-us) Borealis and Ascellus Australis, named for donkeys munching on its hay; the names translate as the northern and southern ass.

A lovely object in itself, the Beehive has the added attraction of lying about a degree north of the Ecliptic. Thus the moon and planets pass between us and the cluster fairly often. On Oct. 1 of this year, Mars will pass through the Beehive. You'll have to look for this before dawn, though, since Cancer will rise at 1:30 am on that day. But that means it will be fairly high before dawn starts to brighten the sky at 6 a.m. It will be well worth an early morning with binoculars or telescope to observe the red planet in this beehive of stars!

Another planet is worthy of observation throughout the summer. The ringed giant, Saturn, rises early in the evening and is visible most of the night. Earth just passed Saturn on the cosmic racetrack of the solar system on Sunday, April 3, so we are now moving ahead of the larger world and as close to it as we ever get, about 800 million miles. Though we are leaving the larger planet behind at thousands of miles per hour, its great distance makes its appearance change fairly slowly, so it won't get noticeably dimmer until mid-summer. However, its position in the sky does reflect the fact that we're passing it. Just as a car you pass on the freeway moves backward with respect to the background as you pass it, Saturn is currently moving "backward" with respect to the stars. That is, it's moving to the west relative to the stars instead of to the east. This is known as retrograde motion and was a source of such fascination for our ancestors that they invented physics to understand it.

With Saturn near the bright stars of Spica (SPY'-kuh) and Porrima (POR'-im-uh), we have the opportunity to observe this cosmic motion. In Figure 1, Saturn is shown in its position tonight. The gray curve shows its motion between now and November. Figure 2 shows its position at various dates, with a zoom-in for its close approach to Porrima in May, June and July. From May 10 to July 14, Saturn will be within a degree (the width of your fingertip held at arm's length) of Porrima. It will be within one-half a degree from May 21 to June 30 and half that distance at its closest approach to Porrima on June 9. This will provide a treat for those with telescopes as Porrima is actually two nearly identical stars, slightly larger and hotter than the sun that orbit each other every 169 years. The distance between them is about that of Pluto from the sun (40 times Earth's distance). From our view of their tilted orbit 38 light years away, they were closest in 2005 and are separating slowly. Currently, only about 3 seconds of arc (1/20th of a minute of arc which is 1/60th of a degree) separate the stars, about one-sixth of Saturn's diameter on the sky (the planet's diameter, not the rings).

On June 14, Saturn stops its retrograde motion to begin its more normal, prograde, motion to the east. It will then move back toward Spica to pass out of the inner portion of Virgo by November. Before that, it will pass behind the sun, aligning with it on Oct. 13, then rise into the pre-dawn sky. By early November, it will be rising more than an hour before dawn, so its continuing motion past Spica can be observed after its closest approach to it, 14.5 degrees, on Nov. 14.

June 14, when Saturn pauses in its motion, is also the day of the earliest sunrise in anticipation of the Summer Solstice, the sun's pause, on June 21 at 5:17 p.m. EDT. That will be the longest day and shortest night of the year. The latest sunset will not occur until June 28, so we have until then to enjoy these lengthening evenings! To review why the earliest sunrise and latest sunset don't occur on the solstice, check out "The Wilderness Above" from Dec. 28, 2010. It's available on the website of the Adirondack Daily Enterprise.

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If you have any questions about astronomy, please visit the Adirondack Public Observatory website at www.apobservatory.org or email Aileen at aodonoghue@stlawu.edu.

 
 

 

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