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Astronomical Observatory: Cool Images


Images: Pluto in Motion

Contents: The image above was made with the Calvin telescope on the night of 13 July 2002. By clicking on the "July 15" button beneath it, you can see how the field changed over two nights. Clicking on "Blink" will toggle back and forth between the two images.

Notice that all stars remain essentially unchanged but one. The one exception, moving from left to right, is no star at all, but the planet Pluto. Its retrograde (westward) motion is a response to the motion of the Earth in its orbit around the Sun. A good analogy is how a person in a field appears to move backward against the backdrop of distant mountains as viewed from a rapidly moving car. The person may actually be walking forward, but so slowly compared to the car that the forward motion is not noticeable. In the same way, slowly moving Pluto (the person) appears to drift backwards against the stars (the mountains) as viewed from the rapidly moving Earth (the car).

At less than one fifth of the Earth's diameter, Pluto is the smallest of our Sun's family of nine planets, It also has the widest and slowest orbit, being forty times more distant from the Sun than the Earth on average, and taking 249 years to circle the Sun once.

Given its tiny size and enormous distance, Pluto appears as an unresolved point of light through most telescopes. It was discovered in 1930 by Clyde Tombaugh, who recognized its planetary nature by seeing its slow drift against the background of stars in a pair of images like ours.

Processing: The images were obtained by Prof. Molnar and by Calvin student Phil Ammar. Each one is the average of five 15 second exposures made with no filter. All were dark subtracted and flat fielded. The first image was made at 11:55 pm EST on 13 July 2002, the second at 10:38 pm EST on 15 July 2002.

Orientation and scale: North is up and East is to the left. The image is 8.7x6.1 arcminutes in size. Pluto is 29.72 AU from Earth, 30.53 AU from the Sun, where an astronomical unit (AU) is the average distance between the Earth and the Sun (93 million miles). (Notice that Pluto is currently much closer to the Sun than the average distance mentioned above. Indeed it has the most eccentric orbit of any planet, greatest ratio of maximum to minimum distance.) Pluto has an approximate visual magnitude of 13.8. This is about 10,000 times fainter than the faintest stars visible to the unaided eye. Pluto's celestial coordinates on 13 July 2002 were 17h0m45.6s,-12d40'34" (epoch 2000). This is in the constellation Ophiucus, which lies between Scorpius and Sagittarius on the zodiac. (The preceding sentence is NOT a typo. Although astrologers like to publicize twelve signs of the zodiac to match twelve months in the year, any real star chart shows Ophiucus as the neglected thirteenth sign.)

Content updated 8/15/02


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