21 January 2017 - Mission Day: 7722 - DOY: 021

The Mercury Transit as seen from SOHO

Caption: These Extreme Ultraviolet Imaging Telescope (EIT) images observed in the 304 Å (upper) and 195 Å (lower) emission lines show the planet Mercury passing in front of the solar corona as seen from the Solar and Heliospheric Observatory (SOHO). The planet is seen as a featureless dark spot inside the green circle on the upper image. The first movie shows the passage of Mercury as observed in the 304 Å emission line. The second movie is a composite of all EIT Mercury Transit observations, in all four emission lines observable with EIT.

The so-called Mercury transit began at 21:15:01 UT and concluded at 22:06:47 UT - almost 52 minutes. In this particular case, Mercury passes across the northernmost edge of the Sun in what's called a grazing transit.

From an astronomical point of view, the event is only mildly interesting for scientists, but it is a unique opportunity for those conducting research through SOHO. For the first time since the spacecraft was launched in late 1995, scientists will be able to take advantage of a Mercury transit to improve the quality of data gathered by SOHO on the rarefied solar atmosphere, called the corona.

The difficulty in obtaining accurate data from coronal measurements is caused by the amount of light coming from the solar disk (much brighter than the dim corona) that bounces around inside the instruments. This scattered light is also referred to by scientists as stray light.

From SOHO's perspective, 1.5 million kilometres from Earth, the closest approach of Mercury to the Sun was at an angle of about 92 arc seconds. The planet passed just above the disk, right in front of the corona. By so doing, the planet intercepted a small part of the coronal light.

Whatever is not black inside the shadow image of Mercury is scattered light inside the instrument. Distinguishing between light from the solar disk and that from the corona will allow more refined measurements of the corona.

Mercury is the planet closest to the Sun. Its orbit brings it to a minimum distance of 46 million kilometres from our star and to amaximum distance of 69.8 million kilometres. Mercury has a heavily cratered appearance, very similar to that of our moon, and has no atmosphere. Temperatures on its surface can reach 400 degrees centigrade.

Instrument: EIT (Extreme ultraviolet Imaging Telescope); Taken: Nov 15 1999, 21:33UT and 21:57UT


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