High-Resolution Image

On November 8, 2006, the orbit of Mercury crossed between the Earth and the Sun, a “transit”. Ingress (between contacts I and II) and egress (between contacts III and IV), when Mercury crossed into and later exited the view of the Sun from Earth, lasted about 2 minutes each, and the total time of this transit was about 5 hours. Mercury transits occur about 13 times in a century. The last Mercury transit occurred in May 2003.

When a Mercury transit occurs, the planet blocks output solar radiation from a tiny portion of the solar disk. This reduction of TSI was monitored by the Total Irradiance Monitor (TIM) for the three SORCE orbits during which the 8 November Mercury transit occurred, although the SORCE was in the Earth's shadow for both ingress and egress. During this latest transit, Mercury decreases the sunlight at the Earth by up to 30 parts per million (0.003%), indicated by the modeled curve in green on the plot. This image shows each TSI measurement (red dots) and averages and associated standard deviation from each orbital observation. It is evident that the reduction in TSI is smaller than the natural variability of TSI. The averages do show a small reduction during the transit, but the statistical significance of these – or the lack thereof – has yet to be determined.

The inserted image at the low left corner shows the planet after the ingress from TRACE. During the transit, Mercury is only about 10 arc seconds in diameter, roughly 1/195th of the Sun’s 0.54 degree angular diameter. Mercury blocked 2.6x10-5 of the area of solar disk. Multiplying the fractional area times the total solar irradiance of 1361 W/m-2 gives a rough estimate of the reduction of TSI during the transit, about 0.035W/m-2.

(Image provided by Greg Kopp (CU/LASP); text by Guoyong Wen, Robert Cahalan, and Greg Kopp)