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New Moon on Christmas Eve

Mark Thompson Astronomy Mark Thompson Astronomy | 12:00 UK time, Saturday, 24 December 2011

Distance travelled ~ 919'776'000 km


Ever wondered why the Moon seems to look different at varying times of the month and sometimes, like today, seems to have totally vanished?


santa at christmas eve

(Image courtesy of Dry Icons - http://dryicons.com)

These are questions that perplexed mankind for centuries but the answer is actually not all that complicated.

what the moon looks now

Image credit: US Naval Observatory/Astronomical Applications Department. What does the Moon look like now?

The first thing to understand is that we see the Moon because it reflects sunlight; turn the Sun off and the Moon would to all intents disappear from view.


While the Earth is spinning and orbiting around the Sun, the Moon is orbiting around the Earth, completing one orbit in 27.3 days. Its actually more accurate to say that the Moon AND Earth orbit a common centre of gravity called the barycentre which lies inside the Earth but not at its centre. Because the Moon orbits the Earth, and the Earth orbits the Sun its easy to see that the actual angle between the three objects varies throughout the lunar orbit and its this variation that leads to the 'appearance' of the phases of the Moon.

At the start of this blog, I stated that we see the Moon because it reflects sunlight. If the Moon lies opposite the Sun in the sky then we see the fully illuminated portion of the Moon and see a full Moon. If on the other hand, the Moon is between us and the Sun then we see the non-illuminated portion and see a new Moon. Then at various points between we see a varying amount of dark and light portions as the phases change from full to new and back again. Today the Moon is at its new phase which means its in line with the Sun and can't easily be seen without sophisticated equipment.

You might expect that during either a full or new Moon, we should experience a lunar or solar eclipse every month (the Moon blocks sunlight reaching Earth during a solar eclipse and the Earth blocks sunlight reaching the Moon during lunar eclipses) but it turns out that the orbit of the Moon is tilted by about 5 degrees to the orbit of the Earth around the Sun. On most occasions at full or new Moon, the Moon is either just above or just below the Sun or shadow cast by the Earth, making eclipses a little more rare.

Interestingly if you measure the time it takes from one full Moon to the next it takes 29.5 days instead of 27.3! This strange effect is seen because the Earth is independently orbiting the Sun and the Moon has to travel a little further to get back to exactly the same angle as the previous full Moon.


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