A line of planets

Ken Tapping, March 7, 2012

In the sky this week…

  • At around 10 p.m. look at that golden star in the east; it is Arcturus, the Herald of Spring.
  • The Moon will be Full on March 6.

Last Monday as I left the observatory, the Sun had just gone down. In the southwest, in a line descending into the sunset glow, was the crescent Moon, and the planets Jupiter, Venus and Mercury. Against that lovely after-sunset blue, it was a beautiful sight to behold. The Moon has now moved on to the east, but that alignment of the planets will be with us for the next week or so. If you have a chance, get out to see it. Find a place without a hilly horizon to the southwest, because any hills block the view of Mercury before the sky has darkened enough for it to be visible. These alignments are striking, and in the past were often regarded as portents. Of course they are merely consequences of the way our Solar System is organized, with the planets moving in more or less circular paths around the Sun, like marbles rolling around a plate. Things are made a little more complicated for us because we see it all from our home on the third marble from the centre.

The planets are not really close together. At the moment Mercury lies about 110 million kilometres away, Venus 122 million and Jupiter 840 million. The Moon is in our laps at a mere 405 thousand.

Mercury is the first planet out from the Sun; Earth is the third. As Mercury moves around its orbit, we see it alternating between being left and right of the Sun. Because it orbits close to the Sun, its separation from it in the sky is always small. We only get to see it easily when it close to its maximum separation, and then only just before sunrise, or just after sunset, when the horizon blocks the glare.

The next planet out from the Sun is Venus. Whereas Mercury is an airless rock ball reflecting only 11% of the sunlight falling on it, Venus has an atmosphere and is enveloped in a layer of white cloud, reflecting 65%. This makes Venus shine more brightly in the sky than anything other than the Sun or Moon. Because its orbit lies inside the Earth’s, like Mercury, we see it swinging either side of the Sun. However having a larger orbit that brings it closer to the Earth, means it wanders much further to the left and right of the Sun. So we see it in a dark sky before dawn, as “The Morning Star”, or after sunset, as “The Evening Star”.  A small telescope will usually show Venus as a glaringly-bright crescent, because as the planet passes between us and the Sun, we see less and less of the sunlit side. Jupiter is the fifth planet out from the Sun. This means we pass regularly between it and the Sun, and that we get to see it in the sky at all sorts of times. That cannot happen with Mercury and Venus. At the moment it lies close to the far side of its orbit, almost on the other side of the Sun.

For planet watching, the situation does not get much better. Although not part of that great Solar System spectacle, Mars, the fourth planet out from the Sun, appears in the eastern sky in the evening and Saturn, the sixth, rises around 10pm. If you have a telescope, do some observing.

We will see another spectacular show on the evenings of March 25thand 26th, when once again the crescent Moon, Venus and Jupiter pose in the southwest after sunset. Mercury though will have sunk back into the Sun’s glare.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton,  BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355

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