The moon will pass by Saturn, Jupiter and Venus in the early morning sky, on its way to a total solar eclipse visible from South Australia in December 2002.

Thousands of eclipse chasers from all over the world will be traveling to the small coastal town of Ceduna, about 800 km (500 miles) northwest of Adelaide, to witness this total eclipse on the 4th of December.  Although our family will not be going to South Australia, we do hope to get some good views here in Sydney, where the partial eclipse will begin 42 minutes before sunset and will grow until it covers 69% of the sun’s diameter and 62% of its surface area.

This eclipse will begin at sunrise in southern Africa, will make its way across the Indian Ocean, where it will be unobservable for most of the day, and will end at sunset in Australia.  Although viewers outside of Africa and Australia will not be able to observe the eclipse itself, the days leading up to the eclipse will nevertheless provide an opportunity to chart the moon’s course as it passes by three ‘‘morning’’ planets visible to the naked eye.

Back in June, many of you enjoyed watching Venus and Jupiter with me in the early evening just after sunset.  But then in July, when I suggested getting up early to see Saturn and Mercury in the morning, the response was less enthusiastic.  One northern hemisphere viewer wrote sarcastically:

Of course, at that time it was summer in the northern hemisphere, and sunrise was at 5 am, which meant getting up at 4 am while the sky was still dark; whereas for us in the southern hemisphere, the sun did not rise until 7 am, so we could ‘‘sleep in’’ until 6 am and still see the planets.

Now, however, the tables have turned, and it is we southern dwellers who must rise early to see Saturn, Jupiter and Venus before dawn, while our northern friends may awaken at a more reasonable hour and still rise before the sun.

Here, then, is the moon’s ‘‘schedule’’ for the days leading up to the eclipse.

The best time to look for the moon and the three planets is from about 2 hours before sunrise until about 1 hour before sunrise.  (See times for selected cities below.)

For viewers in the northern hemisphere, stand facing south.  Venus will be on your left (in the general direction of the sunrise), Jupiter will be straight overhead, and Saturn will be on your right.  Jupiter will be fairly high in the sky (about 65 degrees above the horizon) for northern hemisphere viewers.

Southern hemisphere viewers should stand facing north.  Venus will be on the right (toward the sunrise), Jupiter straight ahead, and Saturn on your left.  All three planets will be fairly low in the sky (Jupiter at about 35 degrees) for southern hemisphere viewers.

Venus is unmistakably the brightest object over the eastern horizon.  Jupiter is also easy to find.  It’s not as bright as Venus, but it’s brighter than any stars nearby.  Saturn, although readily visible to the naked eye, is a bit harder to identify if you’re not sure exactly where it is.  (Right now it’s ‘‘above’’ the constellation Orion in the northern hemisphere and ‘‘below’’ Orion in the southern hemisphere.)

If you were to go outside in the evening and stand in one place all night long, you would see the moon and all three of the planets rise in the east after sunset and move in an arc across the sky until the sun rose in the morning, making it too bright to see them (except that you could still see the moon until it set in the west).

For northern hemisphere viewers, you would be looking south, and the moon and planets would move in a clockwise arc from left to right.  Southern hemisphere viewers would look north and see them move across the sky counter-clockwise from right to left.  (For both northern and southern observers, the motion is east to west.)

The paragraph above describes the motion of the moon and planets in the course of a single night.  During this time, the moon will stay mostly in a ‘‘fixed’’ position relative to the stars and planets.  That is, if it is near Jupiter at the beginning of the night, it will still be near Jupiter in the morning.  However, if you were to go out at the same time each morning, you would see that the planets stay in pretty much the same place from one day to the next, but the moon shifts from one day to the next so that it is closer to Saturn on one morning, then closer to Jupiter a few mornings later, then closer to Venus a few days after that.

As it turns out, the direction that the moon shifts from one day to the next is the opposite of the direction that the moon and all the stars and planets move during the course of a single night.  That is, if you look an hour before sunrise each day, the moon will appear to be further to the east each day than it was the day before.

The motion of the moon and planets during the course of one night is due to the rotation of the earth, which makes all objects in the sky appear to be moving overhead.  But the shifting position of the moon from one night to the next is due to the orbiting of the moon around the earth.

The diagrams below show the shifting position of the moon from one morning to the next, relative to Venus, Jupiter and Saturn.  The diagrams assume that you are looking at the same time each day, perhaps an hour or two before sunrise.  You will see the three planets at the same ‘‘fixed’’ position in the sky on all the days shown.  But the position of the moon will shift so that it starts out closer to Saturn, then moves closer to Jupiter, then passes by Venus, on its way to a ‘‘rendezvous’’ with the sun.

The dates shown on the diagram are for North American and South Pacific viewers.  (They are one day different because of the international dateline.)

Note that for viewers in the northern hemisphere, the moon will appear to be ‘‘above’’ each of the three planets as it passes by, whereas southern hemisphere viewers will see the moon ‘‘below’’ each of the three planets.

The fact that the moon passes slightly ‘‘over’’ or ‘‘under’’ the planets (depending on your hemisphere) explains why there is not an eclipse every month.  A ‘‘New Moon’’ occurs every four weeks, when the moon passes close to the sun in its orbit.  However, it usually does not come directly between the earth and the sun.  Instead, it passes by ‘‘just under’’ or ‘‘just over’’ the sun.  It is close enough to the sun that you cannot see it for a day or two before and after the New Moon; but it is usually not close enough to cause an eclipse.

This time, however, will be unusual.  The moon’s orbit this month is so closely aligned with the sun that the moon will cast a shadow on part of the earth’s surface as it moves between the sun and the earth.

At the times shown for each city, all three planets will be visible (Venus having risen the most recently of the three), and the sky will still be dark enough to see them.

If your city is not listed, you can use the Sky Chart at the Sky and Telescope web site to display a view of the sky for any location in the world at any date and time from the year 1600 to 2400.  (You must have Java enabled in your web browser.)  This is an excellent tool for figuring out what you’re looking at or where to find something in the sky.