Earth & Sky

Moon & Jupiter pair up in sky after midnight

Tonight's Sky for Friday, May 23 2008

Here are Jupiter and the moon, ascending over the eastern horizon after midnight tonight. By Saturday before dawn, you’ll find this bright pair of objects high in the sky, easy to spot by anyone who looks outside.

Friday morning, the waning gibbous moon was in front of the Teapot – a pattern of stars in the western part of the constellation Sagittarius the Archer. In the dawn and predawn hours on Saturday, the moon will have moved eastward through Sagittarius, to couple up with the blazing planet Jupiter. This west-to-east motion of the moon in front of the backdrop stars serves to illustrate the moon’s eastward orbit around the Earth.

The moon always goes eastward through the stars, and so does Jupiter for the most part. But for 4 months out of the year, Jupiter moves in retrograde – westward – in front of the stellar backdrop. This year, Jupiter moves in retrograde – or in a backwards westward direction – from May 9 till September 8. In late August and early September, you’ll see Jupiter right over the handle of the Teapot.

Jupiter’s westward movement is really an optical illustion caused by the Earth in its smaller, faster orbit around the sun bypassing this far distant planet. In the middle of this retrograde – on July 9 – the Earth will swing right between the sun and Jupiter. It’s around this time that the Earth comes closest to Jupiter for the year.

Written by Bruce McClure , 23 May 08