This 259 Kbyte Quicktime
animated clip is a simulation showing the annual variation of the angle of incidence of sunlight on Earth. The point of view for this clip is just above the terminator at 6:00 in the morning (Universal Time), just above Africa. Notice the variation in the angle of shadow along the surface of the Earth. The Sun is at a right angle to this line. It is the variation in incidence angle of sunlight, and not the much smaller variation due to changes in the distance between the Sun and the Earth throughout the year, that causes the climate variations we perceive as seasons.
On May 10, 1994, the central US was treated to a spectacular view of an
annular solar eclipse. Although Bloomington was about 40 miles south of
the path of annularity, the view was still impressive. This 0.3 MB MPEG
clip was made by Indiana University from a set of images taken one minute
apart using a solar telescope at their
Kirkwood Observatory.
In these images from the Yohkoh solar observatory satellite,
million-degree gas in the solar atmosphere shows up as bright X-ray
emitting structures. The curved path of the Moon across the Sun is
due to the orbital motion of the Yohkoh spacecraft.
(Caution: The image file is large, just over 2.4 Mbyte.)
The Yohkoh mission is a collaboration between Japan,
the US, and the UK, with contributions from NASA and ISAS.
For more information on solar eclipses in general and a complete set of path
predictions for solar eclipses in upcoming years, see the
Solar Eclipse Path Predictions page at NASA/Goddard Space
Flight Center.
Chapter 1 RealAudio Files
Find out whether our changing distance from the Sun affects the Earth's temperature.
At aphelion, Earth is farthest from the Sun for the year. Some consequences of
this are explored in this program.
One way to think about planetary orbits is to consider that gravity
is a kind of "force field" that allows any massive thing to capture
passing objects. Some objects that get caught by the Earth will burn
up in the atmosphere (creating meteors, or "shooting stars"), but others
won't get drawn in quite so close. A fascinating example occurred recently
when an object -- apparently a piece of space junk left over from the
Apollo 12 launch vehicle -- was temporarily captured by the Earth's gravity as it passed
near the "First Lagrange point," where the Earth's and Sun's gravities
nearly cancel each other. This large animation (2.1 MByte)
shows the path of object J002E3 as it approaches, orbits, and then escapes
the Earth.
A description of the discovery can be found on the
CNN Website.
This 255 Kbyte Quicktime clip shows the motion of the terrestrial and innermost two Jovian planets from a point of view fixed at about 10 astronomical units from the Sun, above the plane of the Solar System. Things to note include the relative orbital speeds of the Earth and various planets, the elliptical shapes of the orbits of Mercury and Mars, and the noticable changes in distance between any two planets at different times in their orbits. (See also notes below.)
This 271 Kbyte Quicktime clip shows the motion of the same terrestrial and innermost two Jovian planets as above, but from a point of view centered on the Earth. The distance and field of view are the same as in the above heliocentric clip, but in this case our point of view rides along with the Earth, hovering 10 astronomical units above it. This clip shows that the apparent motion of the Sun and observable planets, which is actually due to the real co-orbital motion of the Earth and other planets around the Sun, greatly resembles the system of epicycles and deferents devised by Ptolemy when viewed from a reference system that is centered on the Earth.
For more information on the Ptolemaic and Copernican systems, consider exploring our Chapter 2 Image Archive, which has close-up details of several interesting and historically important documents, as well as portraits of prominent astronomical figures.
Important note: To improve the ability to distinguish the different planets in these clips, a point of view not too far from the plane of the solar system at 10 astronomical units was used, and the frame that you will see compresses 180 degrees of field of view into the rectangular image area. This produces a deliberate "fisheye-lens" distortion of the frame that makes it easier to see the motion and orbits of the innermost planets and compresses the distance between the center of the frame and the outermost planets somewhat.
To see a picture with a more accurate scale for the relative distances between the planetary orbits, see Figure 6.5 of the book.
These movies were made with the program Starry Night. If you are interested in exploring solar system modeling on your own, see our Software Archive.
This 1.1 Mbyte Quicktime movie, also available in MPEG format, demonstrates graphically the conceptual difference between Aristotle's and Galileo's ideas about the nature of gravity. Whether Galileo ever performed this experiment himself on the famous "leaning tower" is highly suspect, but similar experiments had already been done and published by others. Despite these tests, the theory that virtually everyone accepted at the time was that of Aristotle, who believed that heavier objects fall more quickly than lighter ones.
This idea did not seem very likely to Galileo, who instead favored the
theory that heavy or light, all objects released from a given starting point will fall simultaneously with the same acceleration and hit the ground with the same speed, as shown here.
Earth's Seasons Movie
Solar Eclipse in Indiana
Heliocentric Solar System
Geocentric Solar System
Thought-Experiment at Pisa