The 2004 Venus Transit

On June 8, 2004 we had the opportunity to experience a very rare celestial phenomenon. So rare, in fact, that no living person has ever observed it before, since the last transit of Venus happened over 120 years ago, in 1882. We will get another chance again, however, in 2012.

As seen from Earth the planet Venus was visible in front of the Sun for more than 6 hours. From SOHO's perspective, however, 1.5 million kilometres from Earth and slighly off the Sun-Earth axis, the planet Venus did not pass in front of the Sun but glided slowly just beneath the solar disk as shown in the illustration above. However, it was visible against the emission from the diffuse corona. Thus, scientists will be able to take advantage of this Venus transit to improve the quality of data gathered by SOHO.

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

When a planet passes in front of the corona it blocks all of the coronal light coming directly from behind. Whatever light that falls inside the shadow image is scattered light from inside the instrument. Distinguishing between light from the solar disk and that from the corona will allow more refined measurements of the corona. The fiducial "black spot" formed by a transiting planet is also very useful for modelling other optical properties of scientific instruments.

As the planet was making its approach towards the Sun, it came within the range of several instruments that scrutinized it, including some attempting to detect thermal emissions from the planet itself.

The transit was be closely watched by several instruments on board SOHO: LASCO, UVCS, SUMER, EIT, and CDS. Venus was visible in the LASCO C3 field between June 3 & 13, in LASCO C2 between June 7 & 9, and in EIT between 8:21 UT and 19:05 UT on June 8.

TRACE images and movies

The TRACE spacecraft is in orbit around the Earth, so they had essentially the same perspective (and timing!) as ground based observers. TRACE images and movies of the transit are available on this page

Related links

• Once in a lifetime: the Venus transit 2004 (ESA Space Science)
• Venus Transit 2004 (Sun-Earth Connection Education Forum)
• Transit-of-Venus.org.uk
• Predictions for the 2004 Transit of Venus (By Fred Espenak)
• James Cook and the Transit of Venus (Science@NASA)
• SOHO Hotshot: The 1999 Mercury Transit
• SOHO Hotshot: The 2003 Mercury Transit
• The 2004 Venus Transit as seen from SOHO (Technical details)

Picture credits: SOHO/EIT (ESA & NASA)
Instrument: EIT (Extreme Ultraviolet Imaging Telescope)