My Dark Sky

A new year has just started, and the biggest Full Moon of the year is here.

This is no illusion. Some Full Moons really do look larger than others.

A comparison of the Moon taken on two different months in 2007. The right is the biggest full moon of 2007. You can clearly see the difference in the sizes. Click image to enlarge. Credit: thChieh (APGM)

Don’t worry… our Moon does not expand and shrink. This is not due to the physical change in the size of our Moon. Our Moon will always measures 3476 km in diameter (more or less).

Then what causes the change in size?

This question was answered 400 years ago by Johannes Kepler. Kepler explained that the Moon’s orbit around us is not a perfect circle but instead is an ellipse, with the nearest side some 42,000 km closer than the farthest side. We call the nearest point “perigee” and this is where the full Moon will be tonight.

So, this change in size is just the distance between us and our natural satellite. Our Moon will look big tonight just because it is nearer. It will appear 14% bigger and 30% brighter than the rest of the full moon we’ll see later this year.

Do you think you will be able to spot the difference? It is not so easy visually. But if you snap a picture of it and compare with the lesser Full Moons, you may be able to see the difference.

Clear Skies and enjoy the Big Moon tonight!

~ by thChieh on January 10, 2009.

Posted in General Astro Talk, Observations & Tips