My digital camera (Canon XSi with a 300 mm zoom lens) caught Neptune on September 9, 2012.
Neptune was first observed and confirmed in 1846 by
Johann Galle and Heinrich d'Arrest. With a sky of
millions of stars, how does an astronomer know which
star-like spot of light is a distant planet? First
astronomers must know where to look. Urbain Le Verrier
in the preceding several years made careful calculations
from the positions of Jupiter, Saturn, and Uranus
(discovered in 1781), that there must be a small
gravitational perturbation from another planet. Le
Verrier was able to calculate the position of this 8th
planet. Galle and
Arrest, pointed their visual telescopes at the predicted
place for the new planet. They carefully sketched by
hand the relative locations of stars in the field on
different nights and saw that one of the "stars" had moved -
indicating a planet orbiting the Sun and wandering among the
Technology of the 21st century has enabled amateurs to
photograph Neptune and Uranus (both not visible without
optical aid). Digital cameras, particularly those with
exposure control, can photograph the starfields of the night
sky with enhanced sensitivity. Computer software (The
Sky - a commercial product - eventually will become
available for a nominal fee for tablet computers) lets the
user find the location of practically any astronomical
object on any date and time from anywhere on Earth, and
print paper maps. A similar on-line free tool is skymaponline.
Another tool is software that can blink successive
images. A photograph made on another night - after
aligning all the stars to match the first photograph - can
be alternately displayed on a computer screen. The
animation here shows a "blink" of the star field between
Sept. 9, 2012 and Oct. 11, 2012. Similar
techniques were used to track Pluto (PPOW
October 15, 2010). Pluto, however, is about 100
times fainter than Neptune, a telescope was required.
In these pictures of Neptune, the camera was fixed on a
tripod. Exposures of 30 seconds and 20 seconds cause
all the stars' images to show as short streaks due to the
rotation of the Earth.
Due to fall break at Warren Wilson College, there will
be no Physics Photo of the Week next week. Look for
the return of Physics Photo of the Week on November 2, 2012.
Physics Photo of the Week is 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.