SDO Views an Eclipse

One might think it weird to position a Sun observing telescope such that it would experience an eclipse of that celestial body.  However, that is what is happening now to the Solar Dynamics Observatory (SDO) as the Earth moves towards the Autumnal Equinox on September 23rd.  For the next 3 weeks around the equinox the Earth will block out the Sun for the SDO for up to 72 minutes.

So why did NASA decide to do that?  Well it was to put the SDO in a convenient position to be able to transmit data to Earth constantly to the same ground station.  This was deemed to be more important than the ability to monitor the Sun 24/7, especially since the eclipses are not very long.

Similar to many communications satellites, weather satellites and the GPS system, the SDO sits in geosynchronous orbit (GEO).  At that orbit you go around the Earth at the same speed as the Earth rotates which makes you appear stationary with respect to the Earth's surface.  The orbit is at a radius of 42,164 km from the center of the Earth, which is much closer than the Moon.  In order to be truly geosynchronous the orbit has to be inclined the same amount as the Earth's tilt (24.3 degrees) such that it hovers over the equator.

This is tilt means that for most of the year the Earth will not block out the Sun for the SDO. The Earth has only an apparent angular size of 17 degrees from the point of view of the satellite.  Since the inclination of the SDO's orbit follows the Earth's tilt, it is usually far enough away the ecliptic, the plane of the orbit of the Earth around the Sun, when it is behind the Earth not to have the Earth block its view.  This is due to the Earth's north pole tilting towards the Sun in the northern hemisphere's summer, which moves the orbit behind the Earth above the ecliptic, and away during the winter, which moves the orbit behind the Earth below the ecliptic.  This is the same reason why we don't get Lunar and Solar eclipses all the time, because the orbit Moon is inclined with respect to the ecliptic.

However if one is at GEO there are two times of year when the Earth's axis is not tilted away or towards the Sun, the equinoxes.  At those points the orbital section behind the Earth is in the ecliptic and the shadow of the Earth falls on the satellites in that orbit.  Thus the SDO is eclipsed for a short period of time during these sections of the Earth's orbit.  Not that the SDO can't do interesting science during these times. It will have to opportunity to look at the Earth and its surroundings during this time, not to mention that eclipses give a great chance to study the corona of the Sun.

In other news the US Senate has proposed an appropriations bill for NASA which continues funding for JWST.  Thank you to all those who contacted their representatives and senators.  The bill is not final however and still needs to be reconciled with the House version as well as voted on.  So continued support is appreciated.