On the 5/6 June 2012, a very rare celestial event took place that won't occur again until
December 2117 - Venus transited the face of the Sun. For the first time we watched the
transit with a suite of space-based telescopes whilst in situ measurements were made with
ESA's Venus Express spacecraft that has orbited Venus since its arrival in April 2006.
EIS joined in this global effort and captured some stunning data from its orbit around the Earth.
History and modern significance of transits:
The historical significance of a transit of Venus lies in the fact that this seemingly
simple observation unlocked the size of the Solar System in the 1700's. English
mathematician and astronomer Edmund Halley provided a method to determine the Sun-Earth
distance in 1716, based on the effect known as parallax. In its simplest form, Halley's
method used the difference in position of Venus against the disk of the Sun as seen from
different observer locations on the Earth. Today, the transit of Venus provides an opportunity to
study a well characterised planet passing in front of a well-studied star to test
techniques for future work on characterising the atmosphere of exoplanets.
Venus transit science with EIS:
EIS had a few scientific and technical goals in mind in observing the transit: from
looking for the interaction of the solar wind with Venus' atmosphere;
to using the precisely known path of the transit to work out the minute roll of the
telescope's field of view - what you might think of as "which way is up?".
In the movie below, you see Venus first quite far off the Sun's disc to the left
(solar "east"), but it's still silhouetted. This is because the Sun's corona extends
far beyond its surface. Then, we see it going (conveniently!) past a bright active
region -- 2nd and 3rd panels from the left -- and a dark coronal hole -- 4th panel --
before exiting on the solar west.
The wavelength of light that we use here is 195 Å, which is the wavelength emitted
by iron so hot that, not only is it a gas(!) but the atoms of that gas have each lost
11 of their tightly-held electrons. It may be of interest that these images are made
from slot aperture data, whereas we also have slit aperture spectra that will
reveal much more about the precise balance of ions and electrons in the solar
atmosphere and (hopefully) Venus' atmosphere, too.
Movie showing the Venus transit as seen with EIS observations at 195 Å
What's interesting in this movie is that, if you look carefully, you will see the effect of parallax in action as the Hinode satellite changes its viewing position along its polar orbit. Venus seems to
move in little arcs across the Sun. The very effect that inspired Halley almost 300 years ago!
For more information on Hinode's orbit see this website.
Because EIS is a spectrometer it can simultaneously observe the Sun in different wavelengths. The movies below show how the transit looks when seen in a variety of wavelengths of light emitted by the Sun's atmosphere.
Movie showing the Venus transit as seen with EIS at 256.32 Å
Movie showing the Venus transit as seen with EIS at 274.2 Å
Movie showing the Venus transit as seen with EIS at 262.98 Å
Movie showing the Venus transit as seen with EIS at 180.4 Å
Thanks to David Brooks (NRL/GMU), Shinsuke Imada (NAOJ), Toshifumi Shimizu (ISAS/JAXA), Alphonse Sterling (NASA) and Kunichika Aoki (NAOJ), as well as many other members of the Hinode team, for arranging these observations so expertly.