PROBA2 observes an annular solar eclipse
posted: September 16, 2015

This item was written by Matthew West and the PROBA2-team, and can also be read at their P2SC webpage.

The total solar eclipse observed in March 2015 caught a lot of people's attention, especially as the path of totality passed over most of Northern Europe. There was a great deal of fan-fair and plans to observe the eclipse from the ground. However, due to heavy cloud cover, a lot of people had to turn to space-based observations, such as those made by the sun watching extreme-ultraviolet imager: SWAP, on board the European Space Agency's PROBA2 satellite, which images the Sun from the vantage point of a polar Earth orbit, away from pesky cloud cover. More information about the March eclipse can be found here and here.

SWAP observes the solar corona in a passband centered on a wavelength of 17.4 nm. The structures seen in SWAP images have a temperature of approximately 1 million degrees. More information about the SWAP instrument is available here.

It may come as some surprise, especially for those in Europe, that there was another eclipse observed on 2015-Sep-13. Whether you are able to observe an eclipse from the ground depends on your geographic location, in contrast to the March eclipse which was seen from Northern Europe and the Arctic regions, the September eclipse was observed in the southern hemisphere from Antarctica and southern Africa. In any given year the Earth will experience at least 2 solar eclipses due to the Earth and Moon's orbit.

Although the eclipse was not visible from Europe, it was once again clearly observed with the SWAP imager on board the European Space Agency's PROBA2 satellite. An image of the eclipse from SWAP can be seen below.

A movie of the eclipse can be found here, and other media here.

Those with a keen eye for detail, may notice a few subtle differences between the annular eclipse observed last Sunday (13 September; below, left) and the total eclipse from earlier this year (20 March; below, right), the most striking being the apparent size of the Moon. In September, the Moon looked much smaller.

The type of eclipse observed in September was classed as a partial eclipse from the ground, but was observed as an annular eclipse from the perspective of SWAP. Eclipses come in a variety of types, these include:
  • Total eclipses where the Moon completely blocks the Sun's disk
  • Partial eclipses where the Moon only covers part of the Sun
  • Annular eclipses where the Moon's disk covers the Sun, but a significant portion of the Sun's edge remains visible around the Moon
Annular eclipses only occur when the Moon is at apogee, that is, when the Moon is furthest from the Earth. The Moon's orbit is elliptical rather than a perfect circle around the Earth, so from the ground, the relative sizes of the Moon and the Sun vary according to where the Moon is in its orbit. More information about this phenomenon can be found here.

So why does the eclipse appear as a partial eclipse from the Earth, but as an annular eclipse from the perspective of SWAP? The center of the Moon's shadow was located some number of kilometers above (from the perspective of an observer on the ground) the south pole. PROBA2's orbit, at about 700 km above the Earth's surface was the right height to pass through this central shadow region where the eclipse is truly annular. From a vantage point on the ground, however, an observer was not in this central shadow, so the disk of the Moon didn't pass entirely over the disk of the Sun, and the eclipse was a partial one, as illustrated here.

The next total solar eclipse observed from the ground will be observed in 08 March 2016 from South-East Asia and North-East Australia, and then closely followed by another observed in 21 August 2017 from North America.