08 April 2005

The Venus Transit Across the Sun Reduced Sunlight at Earth

On 8 June 2004, a rare astronomical event occurred when Venus crossed the face of the Sun. This event was widely covered in The Citizen Scientist, and you can find news stories and photographs by entering "Venus" and "transit" in the Google search window at the top of the Society for Amateur Scientists (SAS) home page (www.sas.org).

The web has many more stories about the Venus transit, and one theme is about the measured reduction in sunlight that it caused. The measurement was made by a new NASA satellite.

The Solar Radiation and Climate Experiment (SORCE) is a NASA satellite launched on 25 January 2003. SORCE, which is shown in Fig. 1, measures solar radiation, including x-ray, ultraviolet, visible and near-infrared. SORCE also measures total solar radiation.

SORCE has been measuring total solar radiation with high accuracy for more than two years. These data are very important for establishing an accurate value for the solar constant and for global warming studies.

Satellites that measure sunlight have often noted decreases in solar radiance associated with the passage of sunspots across the solar disk. SOURCE measured the largest such decrease ever recorded during October 2003, when the total solar radiance fell by 0.34 percent. This drop is clearly shown in the time series in Fig. 2.

Figure 3 shows the solar irradiance during two sunspot events and during the Venus transit. The reduction during the transit is much briefer than that caused by the long-lived sun spots, which take around 14 days to cross the solar disk if they last long enough for the complete trip.

Figure 4 is an expanded view of the solar irradiance during the Venus transit. During the peak of the transit, solar irradiance was reduced by nearly 0.1 percent. The reduction would have been somewhat greater had Venus crossed the center of the Sun. The crossing occurred near the Sun's limb, which is dimmer than the rest of the Sun. This is the well-known limb darkening effect.

Finally, Fig. 5 shows the data in Fig. 4 along with an image of the actual transit.

Earth satellites like SORCE are an invaluable source of scientific data about the most important star in the universe to residents of Earth. The reductions in sunlight caused by solar flares suggest a variety of studies and possible experiments that can be performed by careful amateur astronomers and modelers. Considerable information about the Venus transit and SORCE is online and is well worth a look.

Forrest M. Mims III

 
Figure 1. The Solar Radiation and Climate Experiment (SORCE), a NASA satellite launched on 25 January 2003. NASA image. Click on image to enlarge.
 
Figure 2. Time series of total solar radiance measurements made by SORCE. Note the significant decline during October 2003 as a large cluster of sunspots crossed the solar disk. Courtesy of the SORCE team. Click on image to enlarge.
 
Figure 3. The transit of Venus across the face of the sun caused the negative spike in solar irradiance shown here. Courtesy of the SORCE team. Click on image to enlarge.
 
Figure 4. This is a detailed view of the reduction in total solar radiance during the Venus transit. This plot is by Greg Kopp of the SORCE team. Click on image to enlarge.
 
Figure 5. Here the data in Fig. 4 have been plotted alongside an image of the actual transit. This figure is from the Climate and Radiation Branch of NASA's Goddard Space Flight Center. Click on image to enlarge.
 
   
Copyright 2005 by Society for Amateur Scientists