Total Lunar Eclipse of 3 - 4 March 2007

The Picture...

After an unpromising day with heavy showers, the clouds cleared by evening and much of the UK was privileged to be able to see the total eclipse in its entirety. It was the first since 2003, the one in 2004 having been clouded out.

The umbral phase, that is, with the Moon either partly or totally within the Earth's shadow, lasted 3 hours 41 minutes, with the first bite appearing in the edge of the Moon at 2130UT and the Moon finally leaving the Earth's shadow at 0111UT. Within this time, the Moon was totally within the Earth's shadow between 2244 and 2357, 1 hour 13 minutes.

I did not bring my telescope to bear on this occasion, though with the view as clear as it was, I wish I had set it up in time. Instead I had to be content with my camera with just a teleconverter lens on the front, and this montage shows the Moon's movement through the Earth's shadow.

Moon moving through Earth's shadow, 3-4 Mar 2007

The Moon moves into the Earth's shadow then out again, but does not disappear completely because it remain lit, albeit dimly, by sunlight refracted by the Earth's atmosphere around the edge of the Earth. The Moon turns red for the same reason as the Sun and Moon often appear red when they rise and set - the long passage of light through the atmosphere scatters the shorter wavelengths, and so makes the sky blue, thus leaving the longer yellow, orange and red wavelengths to travel on back into space and on to the Moon.

Pictures taken at 2132, 2219, 2246, 2331, 2352, 0010 and 0126 UT (GMT) with Minolta Dimage 7 zoomed to 49mm with teleconverter. Exposures range from 1/180s @ f/5.6, !SO100 for the uneclipsed Moon to 4s @ f/3.5, ISO400 for the central eclipse. Pictures also rotated to counteract the effect of the Earth's rotation.

All the photos in the montage above were taken with the same equipment but the totally eclipsed Moon required over 8000 times more exposure than the fully sunlit Moon. It is therefore impossible for one single picture to show both the sunlit and shadowed portions correctly exposed together. The eye too is overwhelmed by the sunlit portion so it is only when the eclipse is nearly total that the red colouring starts to become apparent.

The montage is centred on the Earth's shadow, normally invisible of course, to reveal the Moon's true west to east (right to left) motion against the sky. You can therefore see that the Moon moves by approximately its own diameter (3,476km / 2,160 miles) in an hour. It is only when there is a nearby star or during an eclipse that you can see that this movement is real, for it is normally masked by the Earth's own rotation, which causes all celestial objects to move from east to west across the sky.


The Movie...

The time-lapse sequence below shows the Moon moving through the Earth's shadow, at approximately 600 times the natural rate.

Timelapse view of eclipse over 4 hours

Nearly 4 hours of movement compressed into 24 seconds. Frames were taken at around 10 minute intervals, with the exception of the one at 22:46, where I have slipped in an additional one to show the increasing visibility of the red shadowed portion as the sunlit crescent disappears.

During totality two nearby stars became visible, a red variable star 56 Leonis just below the Moon, and a brighter white star 59 Leonis (magnitude 4.9) ahead of it. The Moon was heading directly for 59 Leonis and I looked forward to seeing it being eclipsed (occulted) by the already eclipsed Moon. Unfortunately, it became apparent that it lay outside of the Earth's shadow, so the event was drowned out by the overwhelming brightness of the Moon returning to full brightness.


The Mechanics...

It was commented at the time that last September's partial eclipse of the Moon took place on the largest Full Moon of the year, with the Moon was at its nearest point in its orbit around the Earth - referred to as perigee. Therefore, by implication, a Full Moon 6 months later will be at the furthest distance from the Earth - apogee.

The Moon's orbit departs sufficiently from the circular to allow the Moon to vary in distance from the Earth from between 356, 410km (221,438 miles) and 406,697km (252,681 miles), a deviation of 14%. Therefore, quite apart from any optical illusion where the Moon appears large near the horizon, the Moon really does vary in size.

Compare the two Full Moons here, both taken with exactly the same optical setup on the camera. The top Moon at apogee is noticeably smaller than the bottom one at perigee. However, the lower picture was taken with the Moon near the horizon, which accounts for the slight compression top to bottom, caused by differential refraction of the moonlight passing through the atmosphere.

Moon at Apogee and Perigee

The Science...

Besides the beauty of an eclipse there is also some useful science that can be deduced from it, a fact not lost on the ancient Greeks.

In the top picture you can see that the diameter of the Earth's shadow is roughly 2.5 times that of the Moon. Therefore, by knowing the size of the Earth you can calculate the size of the Moon and how far away it is. In the 2nd century BC Eratosthenes noticed that the angle of sunlight striking the Earth at noon at a point in southern Egypt and another near the Mediterranean was slightly different, from which he inferred that the Earth was spherical, and calculated a diameter of 12,700km (7,892 miles), very close to the true polar diameter of 12,714km (7,901 miles)

Therefore an initial estimate of the Moon's diameter would be this figure divided by 2.5, that is 5,080km (3,157 miles). However, the Sun is not a point source of illumination so the shadow of the Earth or Moon is not a true cylinder but tapers gradually to a point (see the diagram on this Mr Eclipse page). It was known that the Moon's shadow diminishes to almost nothing by the time it reaches the Earth in a total eclipse of the Sun - this is why the track of totality is so narrow (and in an annular eclipse doesn't reach the Earth at all) - so it was reasonable to assume that a shadow would lose a Moon's width in the distance from the Moon to the Earth.

Therefore, as a first estimate, the shadow that the Moon actually travels through has a diameter of 12,700 less 5,080 = 7,620km (4,735 miles). Divide this by 2.5 gives a Moon diameter of 3,048km (1,894 miles).

Feed these figures into the first calculation again and you get a shadow diameter of 12,700 - 3,048 = 9,652km, which divided by 2.5 gives a Moon diameter of 3,860km (2,399 miles).

Try again and you get 3,536km (2,197 miles). And again, you get 3,666km (2,278 miles). This process of iteration can be continued but you can see that the value is settling down, and although the calculation is very rough and ready it is not too far removed from the true diameter of 3,476km (2,160 miles).

Now hold just a little longer for the distance calculation...

Hold out a small coin so that it covers the full Moon and you find that it is covered when it is held roughly 110 coin diameters from the eye. The law of equal triangles therefore tells you that the Moon is 110 times further away than its own diameter - so multiply up the diameter by 110 and you get a distance of 382,000km (237,000 miles). This lies well within the actual range of 356, 410km (221,438 miles) and 406,697km (252,681 miles).

Clever stuff. However, these calculations simply repeat what was well established by the Greeks over 2,000 years ago, who came up with remarkably accurate measurements in an age long before telescopes, radar, laser rangefinders and space travel.


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