Wednesday, 10 September 2008

Dance of the planets

With Spring has come beautiful clear evenings and mild nights ideal for gazing upon the stars. The western horizon at sunset is particularly lovely at the moment, with a number of planets visible. This is what the Melbourne planetarium skynotes says for September:

A great dance of the planets is occurring in the western sky this month. Bright Venus and faint Mercury can be seen moving higher in the western sky, passing Mars and Spica who are drifting towards the horizon. Near the end of the month, Mercury starts its swing back to the horizon with Spica and Mars in tow.

A dance of the planets -- what a lovely idea! Venus is usually easy to see, bright and beautiful, but Mercury is a planet that's easy to miss, since it's often so close to the sun. I think the next week or two will be the best time for spotting it. Jupiter is also up this month, but starts off quite high, near Scorpius.

Another celestial phenomenon this month is the Spring equinox on 23 September. Here's some more very interesting info from the skynotes:

At 1:44am [on Tuesday 23rd] the Sun crosses the celestial equator and moves into the northern sky. While it is often thought that day and night are equal on the Equinox, this is not the case. It is only the centre of the Sun that is above the horizon for exactly 12 hours; our day is slightly longer at 12 hours and 9 minutes. Why is this? We calculate sunrise and sunset as being when the edge of the Sun first appears above or disappears below the horizon (not the Sun’s centre). What’s more, the Earth’s atmosphere adds its own strange effect – it bends light from the Sun so that at sunrise we happen to see the Sun before it physically crosses the horizon. The reverse occurs at sunset: we continue to see the edge of the Sun for several minutes after it has actually sunk below the western horizon. As a result, our equal day and night occurs before the Equinox on Friday 19th.

Now that I didn't actually know, although in hindsight it makes perfect sense. I feel another celestial celebration coming on . . .

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