"Like a distant peach hanging in the sky!" That's how Akira Fujii described the July 2000 eclipse — a nighttime spectacle that was the most beautiful lunar eclipse the veteran astrophotographer had ever seen.
A lunar eclipse occurs when the Earth lies between the Sun and the Moon, so that Earth’s shadow darkens the Moon. This can only happen when the Moon is full. The dark, central shadow is called the umbra, while the lighter shadow that surrounds it is the penumbra. The penumbral shadow is weak and often difficult to detect; for most observers a lunar eclipse really gets going when the umbra first touches the lunar surface. Totality occurs when the Moon is completely immersed in the Earth’s umbral shadow.
Lunar eclipses get their colorful red-orange hues from sunlight that is filtered and bent by the Earth's atmosphere around into its shadow. This is the light of all the world's sunrises and sunsets ringing the globe at the time. Bright and colorful eclipses occur when our planet’s upper atmosphere is most transparent. Major volcanic eruptions spew dust and aerosols into the stratosphere, resulting in darker lunar eclipses.
The Moon's brightness varies greatly from eclipse to eclipse, which would tell a lot about the state of the Earth's upper atmosphere if only we understood it better. To help in compiling statistics of this kind, many observers rank each eclipse they see on the five-point Danjon Scale.
|Danjon Scale of Brightness|
|0||Very dark eclipse, Moon almost invisible,|
especially at midtotality.
|1||Dark eclipse, gray or brownish coloration;|
details distinguishable only with difficulty.
|2||Deep red or rust-colored eclipse, with a|
very dark central part in the umbra and
the outer rim of the umbra relatively bright.
|3||Brick-red eclipse, usually with a bright|
or yellow rim to the umbra.
|4||Very bright copper-red or orange eclipse,|
with a bluish, very bright umbral rim.
An even simpler technique was advocated 80 years ago by Harvard astronomer Willard Fisher, who graded mid-totality into three classes depending on the equipment needed to see details on the Moon's surface: (1) the naked eye, (2) a 50-millimeter finderscope or binoculars at 7x, or (3) a 150-mm (6-inch) telescope at about 20x. Fisher reasoned that the Moon's surface brightness is unaltered by the aperture, but the visibility of low-contrast features like lunar maria depends greatly on the image scale. The darker the eclipse, the greater the aperture (and power) needed.
Still another useful gauge is the Moon's stellar magnitude at mideclipse. People who wear thick eyeglasses can simply take them off, turning the Moon and bright stars or planets into blobs of about equal size for easier comparison. Looking through the wrong end of binoculars also works.
Photographing a Lunar EclipseThe best advice for photographing a lunar eclipse is to take lots of pictures at many different exposures and throw most of them away. You never know just how bright your subject is going to be.
Most importantly, you'll need a telescope or telephoto lens that enlarges the Moon to a good size. For a typical lunar eclipse, the size of the Moon's image on film equals the focal length divided by 110 (with the result in the same units as the focal length). Thus an 8-inch f/10 telescope, with a focal length of 80 inches (2,030 millimeters), will produce a Moon image 0.73 inch (18.5 millimeters) across. This will easily fit on a frame of 35-mm film, which measures 24 by 36 mm.
An ordinary point-and-shoot camera with a lens of about 40-mm focal length will produce an image of the Moon barely 0.4 mm across, too small to record even as much detail as you can see with your unaided eye. If you have a point-and-shoot camera with a zoom lens, zoom the lens out as far as you can to get the highest possible magnification.
The minimum focal length for getting a good-looking Moon is about 300 mm. With a 500-mm lens and a 2-second exposure you can probably get away with a camera on a fixed tripod, but longer exposures or focal lengths will require a tracking mount to prevent blurring due to the turning of the Earth. A simple, do-it-yourself tracking camera mount is described on page 32 of the May 1996 issue of Sky & Telescope.
The full range of possible eclipse images — from sequences showing how the event unfolds, to close-ups of the shadow's edge on the Moon's surface — calls on a rather large bag of photographic tricks. Compared to recording a normal full Moon with a given camera setup, a lunar eclipse requires exposures perhaps 4 to 1,000 times longer — and everything in between! Much depends on the stage of the eclipse you're trying to record, the darkness of the shadow, and the pictorial effect you are after.
The shortest exposures are useful only during the partial stages near the very beginning and end of the eclipse, and they'll probably bring out little more than the smooth gradation in the shading of the penumbra. That's also what you'll get if you let a camera's built-in metering system take control of the exposure. To record the well-defined edge of the inner shadow, or umbra, which is so striking visually, you'll have to override the system and expose several times longer. Typically that means a ¼-second exposure at f/8 with ISO 400 film, "burning out" the Moon's brightest portion but getting sharp detail near the umbra's edge.
As for totality itself, there is no safe bet — the brightness of the fully eclipsed Moon is too unpredictable. Because film is cheap compared to the rarity of a total lunar eclipse, you can hardly go wrong trying every exposure setting you've got. There's plenty of time to experiment during this leisurely event.
Digital Eclipse Photography
A digital camera, unless it’s a professional model or a digital SLR, is unlikely to have a lens with a focal length long enough to provide a satisfying lunar image. The solution is to shoot through a telescope.
Since most consumer-level digital cameras don’t have removable lenses, the only way to take pictures through a telescope is with the afocal method, whereby the camera is aimed directly into the telescope’s eyepiece. You can hold the camera by hand, mount it on a separate tripod, make or buy a bracket, or use an adaptor to attach the camera directly to the eyepiece.
For recording the partial phases of a lunar eclipse, a digital camera mounted behind a telescope’s lowest-power eyepiece can produce fine close-ups. After overriding the camera’s automatic metering system, you should be able to capture the entire disk of the Moon — the part immersed in the deep shadow core, or umbra, together with that in the vastly brighter penumbra (the shadow’s outer fringe).
The online article "Astro Imaging with Digital Cameras" describes many of the tricks and techniques involved in taking astronomical images this way. But when digitally imaging a lunar eclipse, the key is to shoot, shoot, and shoot some more. The beauty of going digital is the immediacy with which you can see the effects of various settings — and change them! Since a lunar eclipse is a leisurely affair, you can keep on trying until you acquire a good image.