The above chart shows comet 9P/Tempel's (Tempel 1) passage through Virgo over the course of two weeks beginning June 27 and ending July 10 of 2005. Tempel 1's position is marked with a carrot for 0 hours UT (8:00 pm EDT, 7:00 pm CDT, 6:00 pm MDT, 5:00 pm PDT) each day. So, 0 hours UT on June 27 corresponds to 8:00 pm EDT on June 26. The carrot at upper right is for June 27 UT. The lower left is for July 10 UT. The teal line connecting the markers traces the comet's predicted path across the sky. The brightest star in the field is Spica (alpha Virginis), shining at 1st magnitude and easily visible to the naked eye from a typical suburban or rural site. Spica is about 3 1/2 degrees southwest of the center marker along Tempel 1's path.
The above chart covers roughly a 7 degree by 3 1/2 degree patch of sky. Tempel 1's position for 5:52 UT, July 4 (1:52 am EDT July 4; 10:52 pm PDT July 3) is indicated by a carrot. That is the date and time when the Deep Impact probe will slam into the comet at a speed of 23,000 miles per hour (37,000 kph). Astronomers anticipate the comet may brighten from mid-9th magnitude to 6th or 7th magnitude, as light is reflected from the plume of gas and dust ejected during the creation of a stadium-sized crater on the surface of the comet.
Since Tempel 1 will be about 83 million miles (134 million kilometers) from Earth at the time of impact, we won't see any brightening for at least 7 minutes--the time required for light to span that distance--or until about 6:00 UT, July 4. Computer modelling indicates the plume may take a few days to disperse, so the comet may remain noticeably brighter for several nights following impact.
At the time of impact, Tempel 1 will be stationed halfway between two bright stars. 82 Virginis, a 5.0 magnitude M-type star, simmers about 1.3 degrees to the northeast (upper left) of Tempel 1 on the night of July 3-4. Its ruddy hue should allow observers to easily identify 82 Vir. 72 Virginis, a 5.2 magnitude K-type star, shines the same distance to the southwest (lower right) or about 60% the distance from Spica to Tempel 1. Both stars are visible to the naked eye from a dark rural site and should be easily visible in binoculars or your finderscope. Point your scope halfway between 72 an d82 Vir to place the comet within your a low power eyepiece's field of view. Although Tempel 1 may brighten to 6th or 7th magnitude, its light will be spread across a relatively large area. The comet will look like a faint smudge of light against the sky. If the impact produces the desired effect, you'll see a stellar point of brightness within that cloudy glow.
Good hunting.
Bill in Flagstaff
