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Astr110 Photography Projects, Fall 2007

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Neptune and Triton, Tricia Brouwer

Neptune and Triton

Neptune is the eighth planet from the Sun at a distance of 30.1 AU. Its mass is approximately 17 times the mass of the Earth, making it the fourth largest planet in the solar system. Neptune also has a diameter of approximately 49,532 km. Neptune was first discovered by Galle and d'Arrest on September 23, 1846. Galileo had actually observed Neptune two centuries earlier when it was near Jupiter, but at the time he believed it was just a star. Neptune's largest moon is Triton, which orbits 354,760 km from Neptune and has a diameter of 2700 km. Triton's orbit is retrograde, which makes it unique among other large moons.

In this particular movie, you can observe that the orbit of the moon Triton around the planet Neptune is six days. By using Kepler's third law (the square of the orbital period is equal to 4π² divided by the Gravitation constant and the sum of the objects' masses, and then multiplied by the distance cubed), we can determine that the mass of Neptune is 1.02e26 kilograms, which does make it about 17 times the mass of the Earth. To the right, you can see the movement of the stars in relation to Neptune and its orbiting moon. This not only shows that Neptune is a planet, due to its unrelated movement in regards to the star, but also shows that Triton is indeed one of Neptune's moons, as it is clearly orbiting the planet.


Fix, John D. Astronomy: Journey to the Cosmic Frontier. 4th ed. New York: McGraw-Hill, 2006.

Arnette, Bill. "Neptune." Nine Planets. http://www.nineplanets.org/neptune.html

Arnette, Bill. "Triton." Nine Planets. http://www.nineplanets.org/triton.html

Right Ascension Range (J2000) 21:26:40 to 21:26:37
Declination Range (J2000) -15:21:04 to -15:21:18
Filters used red(R)
Range of exposure times Average of 20 images, each 10 seconds the first and second nights. Average of 28 images, each 7 seconds the third night. Average of 40 images, each 3 seconds the fourth night. Average of 60 images, each 3 seconds the fifth and sixth nights.
Date observed

1. 23 October 2007, 1:58:01

2. 24 October 2007, 1:58:01

3. 25 October 2007, 1:57:48

4. 26 October 2007, 3:42:00

5. 27 October 2007, 2:11:55

6. 28 October 2007, 2:48:59