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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Jupiter’s back-and-forth wanderings

On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset as seen from latitude 42 degrees north and longitude 71 degrees west. Chart from StrayyN oghts Pro software. Click for larger image.  .

On October 1, 2009 a nearly full moon joins Jupiter, Uranus, and Neptune in the southeast as shown here about an hour after sunset. (Jupiter is made large to indicate its relative brightness - ut it will look like a very bright star - not a small moon!) This is how the sky appears from latitude 42 degrees north and longitude 71 degrees west. Chart from Starry Nights Pro software. Click for larger image.

The idea here is simple – connect what we can see in the sky this month with what’s actually going on. We’ll do this by watching Jupiter, the easiest object to find right now since it is the brightest “star” fairly high in the southeast shortly after sunset.

With just a few quick checks with binoculars we should be able to track the movement of Jupiter in relation to a bright, nearby star. You should start this project on or before October 1, 2009 if at all possible and plan to observe two or more nights between your start time and October 13. Then observe again in about a week and again near the end of the month.Your first couple of checks should show Jupiter in “retrograde” moving westward among the background stars. Your next two checks should show Juputer has resumed it’s normal eastward movement.

Use the following chart as both your guide and your log. That is, click on it to get a version you can print, take out under the stars, and record your observations on with a pencil.

Click for larger version, suitable for printing.

Click for larger version, suitable for printing.

So why does Jupiter appear to first go one way, then the other? Afterall, it isn’t really doing that, is it? Like the other planets – and us – it’s simply continuing a steady, eastward journey around the Sun. But so are we – and we are moving much faster because we’re much closer to the Sun. So what you are seeing is partly the movement of Jupiter – but also the apparent change in its position caused by our rapidly changing position.

I made the following animation from Solar System Live charts. It shows how Jupiter’s position changes slowly in relation to Earth and the other planets, particularly Neptune. The animation starts with September 1, 2009  and moves a month at a time for six months. The arrow shows our changing view of Jupiter with relation to Neptune, a much more distant – and even more slowly moving, planet. Notice that in late December Jupiter makes another close approach to Neptune – the third this year – which will make especially easy at that time to find this distant and faint planet. Right now you can use the chart above to track it down – it would be just visible in binoculars on a moonless night.

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So let’s review the movements we’re dealing with here.

1. The daily rotation of the Earth causes Jupiter to appear to rise inthe east and move westward as the night progresses.

2. The revolution of the Eartha round the sun at a much higher speed than Jupiter makes it so that for some time the huge planet appears to be moving westward in relation to background stars and the more distant planet Jupiter. That apparent westward motion comes to a stop October 13, 2009.

3. Jupiter’s own motion is more apparent after October 13, as it appears to move eastward against the background stars. This general motion will carry it about 30 degrees eastward – very close to where Uranus can be found now – in about a year. It takes Jupiter almost 12 of our years to make a complete circuit of the sky.

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