Goddard Space Flight Center
Greenbelt, MD
16 November 1999
SOLAR SPACECRAFT HAD GREAT SEATS FOR MERCURY PASS IN FRONT OF SUN
NASA’s Transition Region and Coronal Explorer (TRACE) and the ESA/NASA Solar and Heliospheric Observatory (SOHO)
spacecraft provided a unique perspective on the celestial encounter between Mercury and the Sun Nov. 15.
The transits are relatively rare, averaging only 13 per century. To view it safely requires an experienced solar
observer using proper filters on a telescope or binoculars. The complete transit was visible from most of North
America, where the event occurred shortly before sunset. The total transit was also seen from Hawaii, western South
America, northeastern Australia, and Papua-New Guinea, while a partial transit was visible from the remainder of
Australia, New Zealand and Antarctica. The transit began at 21:15:01 UT and concluded at 22:06:47 UT — almost 52
minutes.
The planet appeared similar to a small sunspot, just like a little dot sailing from left to right. In this particular case,
Mercury passed across the northernmost edge of the Sun in what’s called a grazing transit.
Although transits are not of as burning interest as they were in the eighteenth century, when they were used to
determine the scale of the solar system, TRACE and SOHO will be able to use the observations to obtain better
measurements. For SOHO, the pass will permit better measurements of the Sun’s outer atmosphere, or corona, by
revealing the amount of stray light from the much brighter solar disk that is entering SOHO’s instruments. TRACE is
using the images to resolve the thickness of the solar chromosphere (solar atmosphere just above the visible surface)
with an accuracy never before achieved.
Images will be available Nov. 17 at:
FTP://PAO.GSFC.NASA.GOV/newsmedia/SOHO/MT
Fore details about the Mercury transit, including times and a map of viewing locations, visit:
http://sunearth.gsfc.nasa.gov/eclipse/OH/transit99.html
Fore more about the benefit to SOHO, visit:
*****
[Extracted from the Solar and Heliospheric Observatory website,
http://sohowww.nascom.nasa.gov/]
Hot Shots from SOHO (Nov.16)
Caption: This Extreme Ultraviolet Imaging Telescope (EIT) image [http://sohowww.nascom.nasa.gov/spectacular/]
observed in the 304 Å emission line shows 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. The movies
show the passage of Mercury from 19:48 UT to 23:57 UT, observed in the same emission line.
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, 21:57 UT
