Venus begins 2009 brilliantly

Through breaks in the clouds, Venus is living up to her name “The Evening Star” right now. Find a patch of clear sky about half way up in the sky to the southwest a bit after sunset and you can’t miss her. She is by far the brightest star-like object in the evening sky.

Next week Venus reaches what astronomers call greatest elongation. In this configuration, Venus stands as far from the sun — from our perspective — as she ever gets. And during the weeks on either side greatest elongation, Venus shines brilliantly and sets long after sunset.

Venus is racing to catch up with earth as both planets orbit the sun. The race is like an individual-pursuit cycling race. Did you catch any of those races during the 2008 summer Olympic games? In individual pursuit, two cyclists start on opposite sides of an oval track and race to try to catch up and pass each other.

The race between Venus and Earth started on 9 June 2008 when Venus was exactly opposite the sun from us, in a configuration astronomers call superior conjunction.

Venus has the edge, however, because she orbits on the inside track. Venus orbits closer to the sun than does earth, so she has to cover a shorter distance to get around once. And on top of that, the laws of celestial dynamics make it so Venus always moves faster than earth. So Venus is steadily catching up to us from the west. And boy can we see her coming.

As Venus stands now, the angle between a line extending from earth to the sun and a line extending from earth to Venus is as large as possible. This angle is zero when Venus and the sun are lined up, like they were on 9 June 2008 when Venus was at superior conjunction, and will be zero again when Venus gets directly between earth and the sun in late May.

Venus, now at greatest elongation, is about 47 degrees away from the sun, and sets about four hours after sunset. But as Venus catches up, in the coming weeks, Venus will appear to get closer and closer to the sun and set sooner and sooner after sunset.

It is interesting that Venus will continue to brighten over the next few weeks, as she draws closer to earth, reaching maximum brightness about mid February

Venus’ brightness is affected by two things: how close Venus is to earth, and how much of Venus’ sun-lit half we can see. Right now, Venus is about 0.7 AU from earth (1 AU is the distance between the earth and sun) and appears through a small telescope like a half moon. By mid February, Venus will be about 0.4 AU from earth, but will appear like a tiny crescent moon.

Into the spring, Venus’ will rapidly dim, even though she will continue to get closer to earth. She dims as she draws nearest because as she slides eventually between the earth and sun we are able to see only a tiny sliver of her sun-lit half.

Venus will defeat earth in individual pursuit on 26 March 2009 when she slips between the earth and the sun, reaching what astronomers call inferior conjunction. At inferior conjunction Venus is not visible, even though she is less than 0.3 AU from earth — the closest any planet gets to earth — because then she rises and sets with the sun.

If you can find a patch of clear sky to the southwest in the next few weeks, Venus is brilliant.

This column originally appeared in the Grand Haven Tribune on 9 January 2009.