Physics Photo of the Week
Pluto - Photos by Contemporary Astronomy Students: Max Hunt, Zoe MacLellan, Edward Rubin, Qi
Shen, and Chelsea Ostiguy
by Edward Rubin
On October 6-7, Warren Wilson students helped identify Pluto within the
constellation Sagittarius. Using an 8-inch telescope and deep sky
CCD camera, photos were taken of star fields at the location predicted
by Pluto's ephemeris. As you can see Pluto is very difficult to
see among the brighter stars. Pluto shows up no brighter than
over 1000 stars in the photo. This is because of the low light
magnitude Pluto gives off due to its vast distance from the sun.
Pluto is tiny (about 2300 kilometers in diameter - about 2/3 the
diameter of Earth's Moon) and far from the sun (about 39 times farther
away than Earth); therefore it reflects very little light.
Even when generating an image of Pluto, it is difficult to distinguish
its light from that of other stars. This is why it took two
nights of tracking and careful identification of the star field to find
Pluto. By "blinking"
a photo of the star field on October 6th with a photo of the same star
field on October 7th, we are able to see Pluto’s direct motion on the
celestial sphere. See the animated photo at right.
All planets in the Solar System follow a direct (eastward) motion
around the Sun. Pluto is so far away that it appears relatively
fixed in its position.
Pluto is a dwarf planet located in the section of the Solar System
known as the Kuiper Belt. It is the second largest dwarf-sized
planet (after Eris) in the solar system, and is the tenth largest body
observed directly orbiting our sun. Since its discovery in 1930,
its classification as the 9th planet had been debated by scientists
until 2006, when the International Astronomical Union formally
classified it as a dwarf planet. Pluto's surface is one of the
coldest places in our solar system. Astronomers believe the temperature
on Pluto may be about –375 °F (–225 °C). The planet appears to
be partly covered with frozen methane gas and contains a thin
atmosphere composed mostly of methane. Because Pluto's density is low,
astronomers think Pluto is mainly icy.
In 1905, Percival Lowell found that the force of gravity of some
unknown object affected the orbits of Neptune and Uranus. In 1915, he
predicted the location of a new planet and began searching for it from
his observatory in Flagstaff, Arizona. In 1929, Clyde W. Tombaugh used
predictions made by Lowell and other astronomers to photograph the sky
with a more powerful, wide-angle telescope. Tombaugh's task was to
systematically image the night sky in pairs of photographs taken two
weeks apart, then examine each pair and determine whether any objects
had shifted position. Using a machine called a blink comparator, he
rapidly shifted back and forth between views of each of the plates, to
create the illusion of movement of any objects that had changed
position or appearance between photographs (the exact process shown in
the Physics Photo). On February 18, 1930, after nearly a year of
searching, Tombaugh discovered a possible moving object on photographic
plates taken on January 23 and January 29 of that year. A
lesser-quality photograph taken on January 21 helped confirm the
movement of what is now known as Pluto.
Technology has enabled routine photography of Pluto. Pluto's
orbit is well-studied, permitting an accurate prediction of its
coordinates for any date. The blink comparison is very simple
with appropriate software with digital images. The blink
comparator in 1930 was a cumbersome, very expensive, optical-mechanical
There will be no Physics Photo of the Week published next week, October
22, 2010, due to Fall Break at Warren Wilson College. The next
Physics Photo will be published on Friday, October 29, 2010.
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to firstname.lastname@example.org.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
to see the Physics Photo
the Week Archive.
Observers are invited to submit
digital photos to: