Its all Relative

Kye Ewing - June 17, 2000

With all the solar system showpieces hidden behind the Sun you might think there won't be much in the shallow sky to view this summer and fall. Not so! Like getting out of the constant din of the city where you can hear the crickets chirp, we are now away from the bright "distraction" of the easily seen planets. The next few months actually provide some opportunities that might otherwise go unnoticed if the sights we have become used to were still within view.

Of course, there is the much anticipated Comet Linear S4, which will be passing through the neighborhood this summer. The big question is "how bright will it get". Comets are fickle little things, and when they are making their first pass by the Sun, as it is suspected that this one is, things can changes dramatically for the better or worse quickly and erratically. The latest reports, posted on the Comet Homepage have it at magnitude 9.2 around the middle of May, which is not as bright as some had hoped for.

As this dirty iceberg in space keeps us on the edge of our seats, waiting to see if it will make it to naked-eye status, we have a few more shallow sky targets to enjoy. As luck would have it the minor planet 4 Vesta is at the peak of its brightness range this summer. For a short time each eleven years this remnant from the formation of the solar system becomes bright enough to be a naked-eye object. With an average diameter of 525 km (240 miles) it is the second largest asteroid, but due to its unusual composition including basaltic rock, it IS the brightest.

In 1994 the Hubble Space Telescope observed Vesta while at a distance of 251 million km from Earth. It was found to rotate once every 5.34 hours. The images returned by HST revealed a surprisingly diverse little world, with ancient lava flows and an impact basin so deep that it exposes the inner mantle. In 1960 a meteorite was found that has been identified as a piece of Vesta. The sample has provided many new insights to the composition of this unusual body.

Finding Vesta while it is so bright should be pretty easy. Start from the wide double, Alpha 1 & 2 Capricorni(1) on the western side of Capricornus and swing your view due south about 6 degrees to Sigma(2) and 4 Capricorni. Vesta will be to the west/southwest at magnitude 6.1 to 5.4. It will look like a star, but will be in a little different position from night tonight. On June 26 to 28 it will pass less than one degree from the globular cluster, M75. From a fairly dark location it should be possible to spot it in this otherwise star-poor part of the sky with just your eyes. From the city it can be easily found with binoculars.

While you are in the Capricornus region of the sky, why not stop in on a planet that is not usually at naked-eye brightness but this June and July it IS! Of course, I am referring to the gas giant, Uranus. While Vesta will be at opposition on July 16th, Uranus will follow on August 11th at magnitude 5.7. On December 9th Vesta will pass within 4.5 degrees of Uranus and on Dec. 23rd the gas giant will be only 1.3 degrees from brilliant Venus! Closer to Vesta but not as bright is Neptune. At 20h 35m Dec: -18 25' it is just a little east of the minor planet, but only shining at mag. 7.8.

This is a very interesting scene if you try to keep in mind the distances. Vesta is only about 149,600,000 km (1 AU or astronomical unit) from us. Uranus is 2,832,000,000 km, almost 19 times farther while Neptune is 29 AU's away. The globular cluster M75 is 3.6 billion times more distant at 57,700 light years. (One light year is 63,240 AU's.) Kinda puts the term "shallow sky" into perspective, even when we are talking about such distant worlds as Uranus and Neptune!

1. Alpha 1 Cap
      Distance: 687 light-years
      Magnitude: 4.3
      RA: 20h 18m
      Dec: -12 30'
  2. Sigma Cap.
      Magnitude: 5.3
      RA: 20h 19m
      Dec: -19 07'