Lunar Eclipse - Always Full?

[gsfergy]

I'm waking up today and seeing so many incredible photos taken by those who were lucky enough to have clear skies in North America last night... and it got me thinking (because in Sudbury, Ontario we were clouded out and I had lots of time to think about what could've been a great way to break in my new telescope)... does a lunar eclipse ALWAYS involve a full moon? I have no scientific background here, but my guess is we only see these events when a full moon (or slightly less) is in play. Otherwise, the event could still occur but go largely un-noticed... educate me anyone?!?!

[Ant]

Yes always at the full moon.

The moon is passing through the Earths shadow - that is always in the opposite direction to the Sun. If the Moon passes above or below the shadow you get a usual standard full moon. If it passes through the shadow you get an eclipse.

With a solar eclipse it's the opposite, the Earth passes through the Moons shadow - so that always has to happen at New moon.

Cheers

Ant

[Carina Lass]

Fullest moon you can get - and rubbish for photography. It's like daylight out there!

[Paul73]

An idle follow on thought...

So is it theoritically impossible to get a truely full moon when viewing from earth? Ie, at the point that the moon would be exactly full, earth would be partially in the way?

Paul

[acey]

An idle follow on thought...

So is it theoritically impossible to get a truely full moon when viewing from earth? Ie, at the point that the moon would be exactly full, earth would be partially in the way?

Paul

Idle response: what do we mean by a "truly full" moon? First consider a ball illuminated by a light bulb. If the ball and bulb are very close together then a full hemisphere will not be illuminated. As distance increases, the illuminated portion gets closer and closer to 50 per cent - but to get exactly 50 per cent illumination you would need the distance to be infinite. Similarly, replace the light bulb by an eye, and it is apparent that an absolutely complete hemisphere is only visible at infinity. For the Earth, Moon and Sun the distances are effectively infinite, but not exactly so. The Sun-Moon distance is greater than the Earth-Moon distance, so we must always see a little bit less than the illuminated part of the Moon, which is always a little bit less than half of its surface - assuming the Moon to be a perfectly smooth sphere, which is not quite true on either count.

This calculator works out the visible portion of a sphere at a given distance:

http://www.neoprogrammics.com/spheres/visible_fraction_of_surface.php

The default values are the mean Earth radius and the mean geocentric lunar distance in kilometers, i.e. it calculates the portion of the Earth visible from the Moon, with the answer 49.17%. If you put in the mean Moon radius (1737.1km) it gives the portion of the Moon's surface visible from Earth as 49.77%, when they are at their mean separation. But if the Moon is at its maximum distance from Earth (i.e. its apogee, 0.4055 million km) you get 49.79%. I reckon that would be the theoretical "fullest moon".

Then the question is whether we could see this "fullest moon" without the Earth being in the way and causing a partial lunar eclipse. My guess is yes (given that the illuminated portion must always be a tiny bit bigger than the visible portion), but I don't think we could get an answer without doing a bit of geometry on a diagram, and I don't have an envelope handy...

[Paul73]

Thanks acey

My idle thought process had not got as far as the fact that the sun is larger than the moon and would therefor illuminate > 50% of the moon.

Good answer.

Paul

[Vanilla]

Just for the record: We actually see more than 50 percent of the moon even though it keeps one face toward us. It's call Libration. 

[Paul73]

Just for the record: We actually see more than 50 percent of the moon even though it keeps one face toward us. It's call Libration. 

This is a new one on me! Fascinating.

Wikipedia link to moon gif video :[url=http://upload.wikimedia.org/wikipedia/commons/b/ba/Lunar_libration_with_phase_Oct_2007_450px.gif]

It is amazing how an idle thought and post can lead to learning something genuinely interesting. Firstly, Acey's geometry. Then Vanilla's orbital subtleties.

Paul