This video lesson has the goal of introducing students to galaxies as large collections of gravitationally bound stars. It explores the amount of matter needed for a star to remain bound and then brings in the idea of Dark Matter, a new kind of matter that does not interact with light. It is best if students have had some high school level mechanics, ideally Newton's laws, orbital motion and centripetal force. The teacher guide segment has a derivation of centripetal acceleration. This lesson should be mostly accessible to students with no physics background. The video portion of this lesson runs about 30 minutes, and the questions and demonstrations will give a total activity time of about an hour if the materials are all at hand and the students work quickly. However, 1 1/2 hours is a more comfortable amount of time. There are several demonstrations that can be carried out using string, ten or so balls of a few inches in diameter, a stopwatch or clock with a sweep second hand and some tape. The demonstrations are best done outside, but can also be carried out in a gymnasium or other large room. If the materials or space are not available, there are videos of the demonstrations in the module and these may be used.

This video segment adapted from NOVA shows how Galileo, using his newly developed refracting telescope, observed four of Jupiter's moons, the first astronomical bodies to be discovered since ancient times.

In the early 1600s, most people believed that the Sun revolved around a stationary Earth. This video segment adapted from NOVA tells how Galileo proved that the Sun, not Earth, is at the center of our universe.

This video segment adapted from NOVA shows how Galileo used his telescope to carefully observe and study sunspots.

This video from NASA describes the GLAST satellite, which is equipped with a gamma-ray telescope, and shares some background about the kinds of extreme universal phenomena indicated by the presence of gamma rays.

This video segment from Swift: Eyes through Time introduces and explains theories of the origin of gamma-ray bursts.

This article provides links to web sites about geologic time, fossils, and the archaeology and culture of the Arctic for elementary teachers.

This video segment from NatureScene examines the landforms of Death Valley and how they came to be.

See how the geology of the North Dakota badlands has changed over time in this video segment from NatureScene, featuring the landscape at Theodore Roosevelt National Park.

In this video from Nature, learn about the many different features and effects of glaciers in Ireland.

Have you ever wondered where we are in our own galaxy, Milky Way? "Glitter Your Milky Way" let you get creative while learning the characteristics of the Milky Way and exploring the types of galaxies.

This video segment adapted from NOVA/FRONTLINE examines the greenhouse effect, its role in keeping Earth habitable, and the industrial changes that have led to an increase in the planet's average temperature.

In this video segment adapted from Spanner Films, visit the Alaska Native village of Shishmaref, and learn how an entire town may be forced to relocate because of warmer temperatures, melting sea ice, and coastal erosion.

Students participate in a global campaign to observe and record the faintest visible stars as a means of measuring light pollution in a given location. By locating and observing the constellation Orion in the night sky and comparing it to stellar charts, students from around the world will learn how the artificial lighting in their community contribute to light pollution. Student contributions to the online database will document the visible night time sky.

This article describes a researcher's history of studying sea ice and the dynamic nature of observed changes to the sea ice in the polar regions.

This video segment adapted from NOVA explores the effects of the Glen Canyon Dam on the beaches, wildlife, and vegetation of the Colorado River.

In this video segment adapted from NOVA, a fossil found among the Grand Canyon's rock layers reveals the existence of a shallow sea that once covered most of western North America.

This video segment adapted from NOVA features the youngest rock formations in the Grand Canyon, lava dams, and how they are subject to the eroding power of water.

In this video segment adapted from NOVA, watch residents of the Peruvian Andes build a suspension bridge made entirely of grass. The ancient Inca were a textile society and thus skilled in working with natural fibers including alpaca and cotton. Still, it might surprise people today that their solution to crossing the canyons and gorges of their mountainous empire featured another fibrous material: grass. When you consider how they built a simple suspension bridge, you'll realize that not only was this a practical solution, it was also a safe one. In this video segment adapted from NOVA, watch residents of the Peruvian Andes as they build a traditional and functioning grass bridge the likes of which enabled the ancient Inca people to flourish for several hundred years. Grades 3-12.

Move the sun, earth, moon and space station to see how it affects their gravitational forces and orbital paths. Visualize the sizes and distances between different heavenly bodies, and turn off gravity to see what would happen without it!