Partial Lunar Eclipse of 7 September 2006

Barely Worth a Look...

An eclipse like this would normally barely warrant a mention, being a partial of a mere 19% phase - that is, the Moon hardly entered the Earth's shadow at all. However, the circumstances made it interesting...

Having almost forgotten about the eclipse, I secured a place in my favourite east-facing spot near the Kent town of Westerham on what was a beautifully clear evening, and waited for the Moon to rise...

Harvest Moon Eclipse

I was not disappointed! The Moon cleared the distant ridge of the Chart as a beautiful orange ball with a bit missing...

Eclipse at Moonrise

The eclipsed Moon has just cleared the trees on the distant ridge. The orange colour of the is a result not of the eclipse itself but of the long passage it's light has had to make through the atmosphere to reach the camera, just in the way the setting Sun is red.

The Earth's shadow accounts for the missing portion of the Moon. It too is a very dark red, though at this point the sky was still too light, and the Moon's light has to fight its way through too much atmosphere, for it to be visible.

The Moon's oval shape is caused by differential refraction bending the light as it passes through the atmosphere at this low altitude. This page on Atmospheric Optics explains how this happens - although talking about the Sun the physics is exactly the same and the page has a link to a fine example of Moon distortion taken from the International Space Station

A temperature inversion over the trees on the ridge causes a more local bending of the light that distorts the bottom of the Moon, resulting in the two notches left and right.

The location was indeed well chosen, with a clear view to the south east. As the Moon rose so it brightened, but the eclipse was already at its maximum at 19:51BST, (1851UT), with the Earth's shadow reaching just 19% of the way onto the Moon. The low altitude made the eclipse more interesting because of the associated atmospheric phenomena, and it also caught the attention of more people.

Mid eclipse
A wider view of the Moon at maximum eclipse. My brother, who saw the eclipse from the Midlands, texted me to say the Moon is wearing a toupee - a surprisingly good description perhaps!

5 minutes later Moon was still reddish in colour but by now the gradient from bright to dark from bottom to top was evident. If you were on the Moon you would see the Sun partially eclipsed by the Earth, and the nearer to the shadow you were the more eclipsed the Sun would be. If you were in the shadow itself you would see the Sun totally eclipsed by the Earth, but with a little sunlight bending around the Earth's atmosphere in a reddish ring. This lights the Moon with a dim orange, red or purple glow, a little of which is faintly visible in the photo below.

Partially eclipsed Moon, dark side faintly visible
The Moon rises higher and a very faint illumination of the eclipsed portion is visible. Because the portion eclipsed is so small, a longer exposure should show it better, but in fact it gets drowned out by the light from the uneclipsed part of the Moon.

Shortly after I took this photo the camera's batteries died, and having neglected to bring spares, that put and end to the photos. Anyway, with the Moon rising higher and eclipse now past its maximum, the sight was not as interesting as it was earlier.

However, despite the small magnitude of this eclipse, I was glad to catch this one in such favourable conditions. I am sorry now not to have alerted more people to it, thinking it would be hardly be worth a look, so I hope these pictures go some way to compensate those who missed it!

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