Concordia Base Log
By: Adrianos Golemis, ESA research MD
Log Entry #EXTRA – The
Analemma in Antarctica
Wouldn't
it be interesting to keep a log of your location at exactly 16:00 once a week?
Let’s do that on Mondays: imagine holding up a map of your neighbourhood and
drawing a dot to indicate
your current whereabouts, same time every week. At the end of a calendar
year you could pick up the map and check it: Maybe most Mondays at 16:00 you’re
almost at the same coordinates – at work! But not exactly at the same place;
perhaps one Monday at 16:00 sharp you were at the office next door or you had
to get up from your chair and cross the corridor to answer the doorbell. Maybe
one Monday you left early, so your location indicator showed up at home at
16:00 or maybe it was a free day after all, so your dot was far away, pointing
to the nearest beach. How would it really look, that graph of your exact
position at 16:00 every Monday over a year?
Well,
there is a star who could actually answer that: our Sun! As seen from the
surface of the Earth, the position of the Sun in the sky changes with seasons.
The reason for this is that our planet’s rotational axis has a 23˚ inclination with
relation to the ecliptic (the plane on which all planets revolve around the
Sun) plus the Earth’s orbit around the Sun is an ellipse, not a circle. This
practically means that at exactly the same time, the Sun will be at a slightly
different place in the sky next Monday as compared to this one. If you take a
snapshot of all the Sun’s positions at the same moment over a year, this is
called an “analemma”. Now the difference between the analemma and your own
“graph of location at 16:00 on Mondays” is that the Sun is far more
predictable! It follows the same path in the sky with no deviation year after
year. This pattern looks like the figure 8, only it’s tilted and one of its
lobes appears larger than the other. You can have a look at analemma pictures
here: http://apod.nasa.gov/apod/ap130922.html
What
does change though is how you see the
analemma – and that depends solely on your geographic latitude: A yearly
analemma over Athens is a bit different compared to a yearly analemma of the
same time of day over Paris or Rome. The main change is how much the “figure 8”
shape is tilted and how high above the horizon does an analemma (taken at the
same time from different locations) appear. So how would it look from, say,
Sydney? Well, about the same as from Athens, since their latitudes are almost
inverse. But how would it look from an extremity such as the North or South
Pole?
Having spent one continuous year at Concordia Station, Antarctica and
since my own location at 16:00 every Monday wouldn’t really change at all, I
decided to step up to the challenge of photographing the analemma. To achieve
that, I had to take a photo of the Sun with a special filter not only at the
exact same time once a week, but also from the exact same spot in the Station
which was particularly challenging with the winds and low temperatures outside.
Still, the result was rewarding as it constitutes what seems to be the
first-ever capture of the analemma from within the Antarctic Circle. As you can
see, near the poles only one lobe of the analemma is visible (at Concordia we
experience a 3-months-long polar night) plus the shape appears almost vertical.
Ϡ The Analemma in
Antarctica, music by Vangelis Papathanasiou:
Picture #101: The Analemma in
Antarctica. This picture was selected as NASA APOD (Astronomy Picture of the Day) on
September 23, 2015.
Wait… one last question: how would the analemma look from the surface of
another planet, say Mars? Well, help yourselves: http://apod.nasa.gov/apod/ap061230.html !
Our solar system is full of marvels. ҉
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