Total lunar eclipse of September 16, 1997. Photo: ESA

The new year starts with another eclipse. No need however to retrieve those special sunglasses left over from last August! On the night of January 20-21, we will be offered a total lunar eclipse.

The year 2000 will provide a festival of eclipses. The Sun, Earth and Moon will line up six times. (A situation of conjunction or opposition termed "syzygy"). There will be two total lunar eclipses (the second only visible in Asia, the Pacific and West Americas on July 16) and four partial solar eclipses.

Excepting the partial solar eclipse on July 31 which will be just visible in Scandinavia, the lunar eclipse on January 21 will be the only eclipse this year for Europeans. Depending on weather conditions, the phase of totality should be visible from Western Europe to the western United States.

Orbiting about 150 million kilometres from the Sun, the Earth casts a shadow that on average is 1.4 million kilometres long. This allows our Moon, whose distance from Earth varies between 356,000 and 410,000 km to pass through the shadow when it is in the same ecliptic plane and just at the moment of the full moon.

At the Moon's distance, Earth's shadow has a 10,000 km diameter. This is amply sufficient to envelop our natural satellite and its 3,476 km diameter - even if the Moon does not pass precisely in the centre of the cone, as will be the case this month.

Drawing of how the eclipse works. Photo: ESA

The eclipse will start at 2:03 UT when the left-hand side of the Moon will enter the penumbra of Earth's shadow. Until that time, the light of the full moon will have inundated the celestial dome, allowing only the most brilliant planets (Saturn, Jupiter) and the brightest star formations to be seen.

The first minutes will not be spectacular since the intensity of the residual brightness of the Moon will not allow one to notice the decrease in its illumination. But as soon as half the lunar disk has slipped into the shadow, it will be possible to look steadily at the Moon and see it change .

At 3:01 UT, an hour after the start of the eclipse, the Moon's rim will enter the Earth's shadow on its northwestern side. A further 59 minutes will be necessary for it to slip entirely into the shadow. The total lunar eclipse will last 77 minutes, from 4:05 UT to 5:22 UT.

Contrary to the Moon's shadow on Earth which is eminently dark, even black, the shadow of our planet on the Moon is coloured. 30-40 percent of the lunar disk must be covered before the characteristic reddish hue of a lunar eclipse is perceptible.

The colouring of the shadow stems from sunlight that passes at different levels through the Earth's atmosphere. Only slightly refracted in the higher layers, the light can display yellow, orange, green and blue colours. Passing through the lower atmosphere, the sun's rays are refracted much more and as on a clear evening sunset, offer a far wider pallet of colours.

Lessons from a colored Moon
These dark reddish tints, more or less pronounced, will depend on the precise weather conditions on Earth such as possible thin cloud cover and general atmospheric factors, such as the presence of dust or aerosols in the air. Indeed volcanic eruptions are known to create conditions that allow only a fraction of the sunlight to pass through in which case an eclipse would be much less spectacular.

Precise scientific study of the colours of a lunar eclipse can therefore increase our knowledge of our own planet's environment, for instance about the thickness of the ozone layer.

But one can also learn about the Moon itself by monitoring its infrared thermal emission. The rate at which this infrared decreases during a lunar eclipse is an indication of the cooling of its surface temperature. In turn this rate provides clues on the physical properties of the lunar surface dust.

Artist's concept of SMART-1 nearing the moon. Photo: ESA

Such information from successive lunar eclipses can be compared and may be correlated with data obtained from past lunar exploration missions and future ones such as ESA's SMART-1 orbiter which will be carrying infrared and X-ray spectrometers to map the chemical composition of the lunar surface.

What to look for
As the Moon will be practically at its closest point to Earth (360,000 km), the total eclipse should be relatively dark with a deep red hue. However solar activity, reaching its peak this year, may have its say, influencing the lunar lighting conditions and the Moon could appear a dark brick red.

Amateur photographers will certainly try to get pictures of the well-defined umbral edge at different moments before the totality. Zoom lenses should be used for this, with exposures of typically 1/4 s at f/8 for 400 ASA film. During the 77 minute totality, there should be plenty of time to try different longer exposures.

A lunar eclipse is also a good opportunity to study lunar geography, watching the time when known individual craters enter or leave the umbra.

During the eclipse, hundreds of stars will appear around the Moon. If air conditions are dry and cold and the sky is clear, many of them will be visible to the naked eye or with binoculars.

The pitch-black night will end at 05:22 (UT). The totality phase will be over and a thin silver thread will progressively appear on the southwest limb of the Moon, now just over the horizon. The recovered brightness will lighten the sky, the stars will disappear and the glow of dawn will arrive.

The eclipse will have lasted 5 hours 21 minutes. The next total lunar eclipse, visible in Europe, will take place in less than a year, on January 9, 2001. Not long to wait if the night sky is clouded over this week - or if your alarm failed to go off!