Star Spy: Saturn and Spica

Last updated 05:00 31/07/2012

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On August 2 our full moon will rise in the east just a quarter of an hour after the Sun sets in the west, because it will be on the opposite side of Earth from the Sun.

Since the moon is in continuous motion around Earth, it is at its fullest only briefly as it passes that position.  This time that will actually occur a couple of hours before moonrise on Thursday.

Most of the time when it passes opposite the Sun (once every 29 and a half days) it is in full sunlight in our night sky.  This is because the moon's orbit around Earth is slightly tilted from our orbit around the Sun.

Sometimes, however, it passes behind when its orbit places it almost, or exactly, in a direct straight line with us and the Sun.  On these occasions it moves partly or fully through our shadow. 

Those "full moons" are eclipsed by Earth.  The last lunar eclipse was in June and the next one won't occur until November.

If you look closely at the photograph you will notice that because of the direct sunlight on most of the moon's surface there aren't any deep shadows except around the very edge.  There you can see shadow-filled craters.

If you have binoculars, have a look at the moon, especially around the edge.  Sometimes you can see lighted areas that appear completely separated from the moon. 

These are tall ridges sticking up from the edge of the dark side, high enough to catch the sunlight. If you look closely at the photo you can see such a ridge at just about the 1:30 position of the hour hand on a clock.


Toward the north-northwest just after dark, you have a great view of Mars on the left, with Saturn and its traveling companion Spica above it on the right.  The photo shows their respective positions around 8pm.

They are on their way to set in the west, Mars around 11pm depending on where you are in the country.   Saturn sets about 45 minutes later at the beginning of the week.

If you are watching them from night to night you will notice that Mars is moving quickly eastward in the sky, approaching Saturn, so that by mid-August they will set together.

The rusty red planet, about half our size, is close enough to us that we can see it change before our eyes - well - through our telescopes at least.

Astronomers have photographed many processes familiar to us from our own planet, caught in the act on Mars.  We have observed landslides occurring, and dust devils; dust storms growing in size until they rage over half the planet; thin clouds of water vapour drifting over tall mountains; and polar ice caps that grow and shrink with the seasons. 

It is exciting just knowing that all of these things, and more, are going on up there, changing the face of the planet even while we stand gazing at the small orange dot in the sky.

If all goes as planned, when we look at Mars on Monday night next week, we can know that another of our clever robot rovers has landed there and is preparing to begin its explorations.  Watch this spot.

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