A Lunar eclipse, like the one that took place on the evening of 9th January, is a slow-moving affair. The Sun is behind the Earth and the Moon is on the other side. Slowly and painfully the full Moon advances into the Earth's shadow, goes into totality and then re-emerges and becomes a resplendent full Moon again. Because we have an atmosphere and because of the finite diameter of the Sun (not a point source of light), the shadow of the Earth is never sharp. Nevertheless you can still make out that our shadow is curved, not a straight edge, particularly if you watch an accelerated animation as I am about to show you below. The Moon entered the umbra (the "solid" shadow of the Earth) at 2242 and left it at 0159 Oman time. So what do you do while you are watching this slow-moving event? Take pictures, of course
J But how? Let us start with some key observations:
We have a full Moon (that is fully illuminated by the Sun) at the beginning. Basically, it's a landscape (OK, Moonscape!) in bright sunshine; so there is no shortage of light. Exposure can be pretty brief, even if we use a small aperture telephoto lens. Rule of thumb says that a sunlit landscape requires an exposure equal to the reciprocal of the ISO speed of the film at an aperture of f/16. ISO 400 film would require 1/400th sec at f/16. So for the period before the eclipse we can use any film, fast or slow, and still expect reasonably sharp photos. With a long telephoto lens and slowish film one would still need to use a tripod though. Another rule of thumb says that a hand-held photo will very likely show camera shake if the exposure is longer than the reciprocal of the focal length in mm. Ie. To handhold a 500mm lens the exposure needs to be shorter than 1/500th of a second. But I wanted to use a 2000mm lens since that focal length fills a 35mm frame very nicely with the Moon's image. No sensible person handholds a 2000mm lens!
We then get an eclipsed Moon. During totality the Moon goes very dark, and is illuminated only by the light that gets scattered diffusely through the Earth's atmosphere. But how dark does it get? Of course, it's difficult to guess ahead of time. But it will be a LOT darker. By the way, in this case totality lasted from 2341 to 0051; not a quickie one or two-minute affair like for a Solar eclipse. Consequently we'd better use a very fast film for the period of totality. To simplify life I opted for Fuji ISO 800 colour print film throughout the exercise. And made sure that the telescope was tracking the Moon accurately, since I was unsure as to how long the exposures needed to be during totality.
We also know that if the photo exposure is set correctly at each step, eg. just leaving a camera to set the exposure automatically, then the resultant pictures would print equally bright (or equally dark) regardless as to whether the Moon was in totality or shining in full glory before the eclipse. This would of course give a wrong impression as to what really happened. It is therefore best to use a fixed exposure throughout. But if we did so, then the pictures during totality would be pitch black! Consequently I opted for two fixed exposures, one during the eclipsing phase (one thousandth of a second) and another during totality (4 full seconds for each photo). The 1/1000th was obtained by the camera's meter before the eclipse began, the 4sec was also obtained by the meter after totality had commenced. I'll show you the two sets separately. Our eyes adjust very quickly and we can clearly see the reddish, fully-eclipsed Moon, even though it is very much dimmer. In fact, according to the meter in my camera, 4000 times dimmer.
The photo below, taken while the eclipse was progressing, trying to use a compromise shutter speed, illustrates the problem of exposure:
This photo shows the still sun-lit crescent as way over-exposed but does illustrate the reddish tone of the eclipsed part. The animated gif below is a sequence of photos taken at 15 minute intervals, but the ones during totality have been left out (they appear pitch black). Actually, post-totality there were clouds, and I could not stick exactly to my 15-minute periods but had to make do with whenever the clouds cleared.
Note that the Earth's shadow appears to be off-center, otherwise the Moon would have entered and left it in a symmetric manner. North is at the top and one can see that our shadow is centered somewhere south of the Moon. Also note how huge our shadow is compared to the Moon itself. Exposure for each photo was 1/1000th sec using a Celestron Ultima 2000 8" Schmidt Cassegrain telescope at f/10 (focal length 2030mm). The scope was mounted on an equatorial wedge and tracking at the "Lunar" rate. The gif below shows what happened during totality (each photo had a 4sec exposure):
Note that the glow is lopsided and is moving from top-right to top-left, again because the Moon was not exactly in the center of the Earth's shadow. The glow here is NOT the sunlit crescent. At 1/1000th sec the whole Moon photographs as pitch black.
By the way, the very first photo was taken at 2215, before the eclipse began at 2242. But being so close to an eclipse, the Moon was as fully illuminated as it ever gets from our point of view. You can see an enlarged image of the full Moon, together with the locations of the Apollo landings by clicking
