Three of the brightest planets, Venus, Mars, and Saturn, are gathering in the western sky just after sunset. This week, the slender crescent moon pays each planet a visit in turn.
On Wednesday night, July 14, the moon will be directly below the brilliant planet Venus. If you look at Venus through a telescope, you may be surprised to see that it looks like a fat gibbous moon, 65 percent illuminated, while the moon is a slender crescent just 15 percent illuminated. How can this be?
It is because Venus is on the far side of the sun, so that it is ?front-lit? whereas the moon is between us and the sun, so is ?back-lit.? Think of the four objects forming a ragged line: Earth, moon, sun, Venus, with the sun somewhat off to the right.
But the moon's encounter with Venus is the just the beginning of its planetary tour this week. (This graphic shows how to see the moon as it visits Venus, Mars and Saturn).
By Thursday night, the moon will have moved in its orbit around the Earth so that it is directly below Mars, which should look distinctly orange-red to the naked eye. Mars, too, is on the far side of the sun, and very tiny in even the most powerful telescope.
Finally, on Friday night, the moon will have moved to the position shown in the image, to the lower left of Saturn. Saturn has been a disappointment this year to telescopic observers because its famous ring system has been almost edge on to our view, and so almost invisible.
Notice how the moon is significantly below the ecliptic, the path taken by the sun in its apparent travel through the sky, while all three planets are significantly above the ecliptic.
This is a powerful demonstration of how the moon and planets do not all travel in the same plane. Their orbits are all slightly tilted relative to one another.
The orbit of the moon is tilted more than those of the planets, which explains why it lies so low relative to the ecliptic, and why it so rarely passes directly in front of any of the planets.
Although you may not notice it over the next few nights, the three planets are themselves drawing closer together, heading towards a triple conjunction in the first week of August.
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This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.