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Aphelion – July 4th July 3, 2009

Posted by baldricman in astronomy.
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Tomorrow, July 4th, the Earth will be at it’s furthest point from the Sun, in it’s eliptical orbit. This is known as “Aphelion” (with the closest point known as the Perihelion).

Orbits of the Inner Planets

Orbits of the Inner Planets

The effect? Well, contrary to what you may think, this does not really effect our seasons in any meaningfull way. The seasons are, of course, created by the tilt of the Earth on it’s axis, in relation to the Sun. However, the Sun will appear slightly smaller in the sky, by roughly 3% (No, you won’t notice that)

This happens once a year, at more or less the same time, and the following table gives the date and time (GMT) of each aphelion. (Table from wikipedia page here)

Year Perihelion Aphelion
Date Hour Date Hour
2007 January 3 2000 July 7 0000
2008 January 3 0000 July 4 0800
2009 January 4 1500 July 4 0200
2010 January 3 0000 July 6 1100
2011 January 3 1900 July 4 1500
2012 January 5 0000 July 5 0300
2013 January 2 0500 July 5 1500
2014 January 4 1200 July 4 0000
2015 January 4 0700 July 6 1900
2016 January 2 2300 July 4 1600
2017 January 4 1400 July 3 2000
2018 January 3 0600 July 6 1700
2019 January 3 0500 July 4 2200
2020 January 5 0800 July 4 1200

You can see a good representation of the difference in the apparent size of the Sun, in the form of a composite image (from 2 photos, one at Aphelion, one at Perihelion), on the NASA site over here.

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Comments»

1. Allen - July 3, 2009

Interesting stuff!

2. carelj - August 17, 2009

Funnily enough I always thought the aphelion happened during December, which is why Northern Hemisphere winters seem colder than the Southern. But I guess that reasoning quickly falls apart. E.g. why are Southern Hemisphere summers generally hotter than Northern Hemisphere summers. So, in short: thanks for enlightening me!

baldricman - August 17, 2009

Hey Carel, thanks for commenting. As far as I understand, our seasons aren’t affected by apheliona and perihelion, at least not noticeable…. but I actually can’t think of why southern hemisphere summers would be warmer, as you say. Perhaps the lower land mass in the the south? (I know the oceans are important in this regard, but don’t know exactly *how*…)


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