No need
however to retrieve those special sunglasses left over from last
August! On the night of 20-21 January, 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 16 July) and four partial solar eclipses.

Excepting the partial solar eclipse on 31 July which will be just visible
in Scandinavia, the lunar eclipse on 21 January 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.

Alarm clocks required

The eclipse will start at 2:03 UT (3:03 CET) 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% 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 coloured 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.

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 9
January 2001. Not long to wait if the night sky is clouded over this
week – or if your alarm failed to go off!