High-Resolution Image

On June 8, 2004, Venus crossed between the Earth and the Sun, a "transit", for the first time since 1882. Ingress and egress times, when Venus crossed into and later exited the view of the Sun from Earth, lasted about 20 minutes, and the total time of this transit was about 6 hours. During ingress, the SORCE Total Irradiance Monitor (TIM) measured an associated decrease in the solar energy arriving at Earth, termed the total solar irradiance, or TSI. The lower value was maintained as Venus transited the Sun, then a similar increase returned the solar energy to normal after egress. In agreement with predictions, at ingress the Sun's energy dropped by about 1.36 Watts for each square meter of Earth's cross-section, or slightly less than 0.1%, or 1000 parts per million (ppm), of the TSI.

The data shown here, which have been adjusted to the mean Sun-Earth distance of 1 AU for comparison with other TSI values seen throughout the year, shows a decrease from the Sun's normal TSI value of 1361 W/m2 down to 1359.7 W/m2, as predicted from model computations. The gaps spaced throughout the data occur at times when the SORCE spacecraft was in the Earth's shadow and could not view the Sun. As luck would have it, the ingress was recorded, but egress occured during a gap when the Sun was not visible by SORCE. Venus did not move across a diameter of the Sun, but rather across a shorter chord near the Sun's south pole. The slight curvature in TSI during transit, bright to slightly darker and then brighter, as Venus moves along the chord of the Sun is due to the fact that the Sun is not as bright near its edge, or limb, and Venus therefore blocks less radiation than it does as it moves further onto the solar disk. This is a well known effect called Òlimb darkeningÓ on the Sun. The small fluctuations (about 50 ppm) in brightness on short time scales are from normal solar oscillations and can be seen both before and after the transit.

Inset in lower left of the plot is an image of Venus during ingress (5:34 UT) taken by TRACE. Venus on the eastern limb of the Sun shows a faint ring around the planet. This faint ring, with a brightness of only about 1% of the brightness of the Sun close to its edge, is a consequence of the scattering that occurs in the atmosphere of Venus, allowing some sunlight to show around the edge of the otherwise dark planetary disk.

[Gary Rottman, SORCE PI, Greg Kopp, TIM Instrument Scientist, Bob Cahalan and Douglas Rabin, SORCE Project Scientist and Deputy Project Scientist. Thanks are due to the entire SORCE Team, at UCO/LASP, Orbital Sciences, Goddard Space Flight Center, and other participating institutions in the USA and Europe.]