March 14, 2008
A Missed Opportunity?
Left: Lunar eclipse seen looking back at the Earth from the Moon -- Surveyor III from Section 2b of NASA SP-168 (reversed and rotated for symmetry) Right: Lunar eclipse seen looking at the Moon from Earth -- George Tarsoudis
To an observer on the Moon, the Sun appears about the same size as we are accustomed to seeing the Moon in our own sky. The Earth is about four times larger in diameter. During what we perceive as a lunar eclipse, an observer on the Moon, situated in the umbra of the Earth's shadow, and looking back towards Earth, will see the Sun's disk completely covered by the Earth's; however, the lowest 10-20 km of the Earth's atmosphere can bend (refract) the sunlight enough to see at least a part of the geometrically hidden Sun, sparkling as through a prism. This refracted sunlight, reddened by its passage through the Earth's atmosphere, has long been assumed to be the source of the copper-red hues seen, at lunar distances, even in the core of the shadow, as in George's wonderfully atmospheric view of an eclipsed Moon seen through thin clouds. Yet the exact details of where the red light comes from, and why it varies in color and intensity from eclipse to eclipse, have never been adequately documented. The best we have is this grainy little image serendipitously obtained from Surveyor crater in 1967. At the moment the images used to construct this color composite were obtained, the entire Sun was well hidden behind the Earth's dark disk, but it was closer to the edge in the upper right, where a relatively small amount of refraction allows bright sunlight to stream through. Elsewhere around the Earth's limb, the Surveyor III camera detected numerous beads and patches where some combination of increased transparency and/or anomalously great refraction made the Sun visible. But this is almost certainly not an accurate picture of what the hidden Sun looks like. One can only assume that if taken with higher resolution and better exposure, all the light visible from the Moon would be seen to originate in the paper thin lowest layer of the Earth's fragile atmosphere and probably be of more intense and varied colors -- a sort of combination of all the sunrises and sunsets on Earth seen at one moment.
On February 21, 2008 (UT), for the first time in the more than 40 years since Surveyor III, two active imaging satellites were situated at the Moon during a lunar eclipse and had the opportunity to witness this spectacular phenomenon. Although spacecraft are normally designed to work on battery power when their solar cells are in shadow, China's Chang'e-1 spacecraft was, with some suspense, powered down into a kind of safe-mode for the duration of the eclipse, project scientists later proudly announcing their ship had successfully survived the outage. Whether Japan's Kaguya spacecraft, part of whose stated mission is to return HDTV images of the Earth as seen from the Moon, attempted any photos during February's eclipse is unknown; but after nearly a month no images have been mentioned or released. Kaguya's HDTV camera could certainly have returned images clearer than those from Surveyor III; and the Terrain Mapping Camera, if it could have been pointed towards the Earth's limb, should have been able to confirm the thinness of the layer from which the light originates (and although it could not quite resolve it, would almost certainly have revealed unexpected details).
Perhaps there are technical difficulties that prevent this kind of imagery with the present satellites, but one wonders if during the upcoming eclipse on August 16 anyone on the Moon, curious about what is blocking the Sun, will look back at Earth?
- Left: Surveyor III obtained 20 TV images, with various filters and exposures, during the April 24, 1967 eclipse. The small color image is a composite of vidicon readouts taken through red, green and blue filters at around 11:24 UT. The left hand side of the image is probably dimmed by vignetting.
- Right: CCD image by George Tarsoudis; March 3, 2007, exact time unknown. LXD-75 8" SC, Canon EoS 350d at prime focus with focal reducer f/6.3. Moon: ISO 400 TV 1.6sec. Clouds: ISO 200 TV 1/40
- A 1968 article detailing the Surveyor eclipse observations can be found on-line as pp. 50-57 of NASA SP-146 (179 MB PDF) and as pp. 119-126 of NASA SP-184 (31 MB PDF).
- The camera used to obtain the Surveyor III eclipse photos was returned to Earth by the Apollo 12 astronauts, who themselves witnessed an occultation of the Sun by the Earth on their return journey -- but from a much closer distance than that which produces the colors seen on the Moon. Their excitement at witnessing this event, described as the most spectacular sight of their entire trip can be experienced by reading hours 240:33:39 - 241:49:08 in the transcripts of the Apollo 12 Flight Journal.
- George's website
- Chang'e-1 website (coming soon?)
- Kaguya website
- NASA's lunar eclipse predictions: for the August 16, 2008 eclipse, at around 21:00 UT, any spacecraft situated over the southern two-thirds of the Moon's nearside will be in the umbra of the Earth's geometric shadow, and therefore see the Sun totally eclipsed by the Earth.
- For a highly technical theoretical discussion of how the details of the Earth's atmosphere are expected to impact the expected appearance of the Sun as seen from Moon during an eclipse, see the works of Frantisek Link, in particular his 1969 book and 1972 review article.
Yesterday's LPOD: The Great Migration
Tomorrow's LPOD: The Lunar South Pole
(1). That is a very good point about the current lunar orbiters returning images during totality (I had assumed that either the maneuvers to image earth may have been too complicated at the time of the eclipse or that there would be a danger pointing the cameras towards the sun / earth and deemed too risky early on in the missions). If we don't see any images forthcoming then maybe we will during future eclipses. Also, it's very enlightening to see that Surveyor image... I had no idea that this had even been attempted, thanks Jim.