The eclipse in this week’s guess the planet image is quite
similar to the one which was visible from some parts of the world on Monday..
The main difference is that in this image, it is the Earth which is blocking
out the sun. The observer is not on the moon, but rather in orbit around it. This
image was captured by the Kaguya spacecraft of the Japanese Space Agency’s
(JAXA), when the Earth passed between it and the sun. A film of this eclipse, including
a spectacular diamond ring effect can be found on the JAXA website.
Eclipses have been observed since antiquity and our ability
to predict them has always been very good. We know the movements of the planets
with great precision, and can easily project those movements into the future.
There are few sources of uncertainty which can affect these motions, so phenomena
such as eclipses, the recurrence of comets, and the movements of the planets across
the night sky can all be predicted quite reliably. This is also true of
periodic meteor showers such as the Perseids and Leonids which occur when the
orbit of the earth intersects fields of debris, generated by the passage of
comets. As a comet moves along its orbit it leaves debris behind. This means
that whenever the earth crosses the comet’s path it hits some of this material,
even if the comet itself is a very long way away at the time. This debris burns
up in the atmosphere producing shooting stars, which can always be seen in the
same parts of the sky at certain times of year.
We have quite a few images of eclipses from other planets.
The image below was captured by NASA’s surveyor 3, from the surface of the moon
in the 1960s. The image isn’t as high quality as the more recent observation
from Kaguya, but is still an impressive example of how our home planet can
block out the sun.
The image below was captured by the New Horizons spacecraft
as it passed Pluto. Here the dwarf planet is silhouetted against the sun. At first
glance this might not seem like a particularly useful image, after all none of
the surface of the small world is visible from this angle. However, it wasn’t the surface which the New Horizons team were
interested in when they took this image. The blue glow around Pluto is its
atmosphere, and from this angle the light of the sun passes directly through
the “limb” of the atmosphere, allowing measurements of its properties to be
captured.
You can learn a lot about the chemical composition of a
material from the way light interacts with it. Different elements
preferentially absorb and reflect light of different wavelengths. This is why
materials appear to be different colours, and looking at the colours, or
regions of the spectrum, that interact with materials is a good way to determine
their chemical makeup. This process is called spectroscopy and can be used at a
range of scales. On the very small scale tiny amounts of material can be
superheated to see what wavelengths of light they emit. On the very large scale
the wavelengths emitted by stars and galaxies can be analysed to look for peaks
in certain parts of the spectrum which indicate their chemical composition. Other
materials won’t emit light, but will reflect or scatter it, and again some
colours of light will be reflected more strongly than others. This is the case
in this image of Pluto’s atmosphere.
Examining the interaction of light with the atmosphere
revealed a lot of information and allowed the New Horizon’s scientists to work
out how Pluto’s haze probably formed. NASA’s description of this image states
that: “…the haze is a photochemical smog resulting from the action of sunlight on
methane and other molecules in Pluto's atmosphere”. One interesting thing about
this image is the blue colour. On Earth we are used to seeing a blue sky, and
it appears that colour because our atmosphere preferentially scatters light
with a blue wavelength. The new Horizons team note that the same is true of the
particles that form the haze on Pluto leading to this characteristic colour.
Image Credits:
Japanese Space Agency (JAXA) (C) JAXA/NHK, http://space.jaxa.jp/movie/20090218_kaguya_movie01_j.html
NASA/Johns Hopkins University Applied Physics
Laboratory/Southwest Research Institute https://photojournal.jpl.nasa.gov/catalog/PIA21590
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