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 Ask a scientist Not up to speed with science jargon but have a sciency question?

#11
1st February 2016, 12:28 PM
 cyclist And if I can reach the pedals, I'll be sweet Join Date: Aug 2010 Location: Sydney Posts: 3,062
Re: The moon illusion

Additional to the fact that the earth's orbit isn't circular, the sun is not stationary. It is affected by the gravity of the planets (mainly Jupiter and Saturn, but the others will still have a small impact). Because of this, if the earth is in the same position of it's own orbit, it will have a different distance to the sun depending on where Jupiter is in it's orbit.

They used to use this technique to discover exoplanets, as distant stars would "wobble" as their planets orbited.
#12
1st February 2016, 12:32 PM
 two dogs Why do you ask ...? Administrator Join Date: Jan 2009 Location: Shell Cove, NSW Posts: 9,237
Re: The moon illusion

Quote:
tmorg said
The next full moon is February 23. I am going to measure this thing with a coin if I can get organised enough haha.

Quote:
 Unrelated to the original question, but the apparent size of the moon does vary, due to its elliptical orbit.
I read that the moon's orbit is nearly circular.
I've just looked it up, and it has an eccentricity of 0.05, which is isn't all that elliptical. There's only a 1,200km difference between the major and minor axes (768,800km and 767,600km respectively).

The apparent difference in the size of the moon is primarily due to the moon orbiting around the earth at one of the two focii of that ellipse, which are offset about 20,000km from centre of the ellipse, resulting in variation of around 42,000km between the perigee (closest distance, around 363,400km) and apogee (furthest distance, around 405,500km).
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