Universe Cycle - Solar System (3)

  • Discovering the terrestrial planets.
  • Exploring the rotation of the inner planets.
  • inner planets
  • revolution
  • rotation
  • terrestrial planets

Students make a model of how the inner planets orbit around the Sun.

terrestrial planets - Mars, Earth, Venus, 
Mercury, and Pluto


The Solar System consists of our Sun (a star) and an assemblage of smaller bodies that revolve around the Sun. The smaller bodies include the planets and their moons, asteroids, comets, and interplanetary dust.


The major planets, in order of increasing distance from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Scientists commonly divide these into two groups. The terrestrial planets (also called inner planets), Mercury, Venus, Earth and Mars and the gas planets of Jupiter, Saturn, Uranus, and Neptune. Pluto is considered a terrestrial planet, but not an inner planet.

Moons, or natural satellites, are bodies that revolve around a planet. The Earth has one moon, while the other planets have none to many. Moons have varying compositions. The Earthís Moon is essentially rock. Other moons, particularly those of Jupiter and Saturn, appear to have rocky cores, but are surrounded by gaseous atmospheres.

Asteroids are small bodies of rock and frozen gas. They tend to be concentrated in a belt between the orbits of Mars and Jupiter. Some asteroids also orbit the Sun, on paths that cross the Earthís orbit. These objects began in the asteroid belt, but were pushed into orbit around the Sun by the gravitational effects of Jupiter, or collisions with other asteroids. Meteors or "shooting stars" refers to flashes of light that dart across the night sky. These occur when an asteroid enters the Earthís atmosphere and burns up.


A comet is a small body composed of frozen gas and small amounts of rock. Comets are leftovers from the formation of the Solar System. Most comets reside in a belt outside the orbit of Pluto., or much further away in a huge shell surrounding the Solar System. Comets travel inward toward the Sun, developing elliptical orbits. As a comet approaches the Sun, solar radiation heats it up. Escaping gas and dust form the cometís tail.

Mercury is the closest planet to the Sun. It orbits the Sun quickly, once every 88 days. Because it is so close to the Sun, it is difficult to see. Mercuryís orbit is very elliptical, ranging from 46-70 million km from the Sun. It rotates slowly once every 59 days. The Sun appears 2.5 times larger from Mercury than it does from Earth. Surface rocks facing the Sun roast to 400oC while the long night surface cools to -170oC. Mercury is small, about 4850 kilometers (~3000 miles) in diameter. The surface of Mercury looks like the Moon, but is gray-orange in color. There are many impact craters and large areas of lava, like the Moon's large plains.

Venus the second planet away from the Sun, is Earthís closest neighbor. It is about the same size as the Earth, a little over 12,000 kilometers (7300 miles) in diameter. It is sometimes considered to be the Earthís twin. Venus rotates counterclockwise once every 242 days. This is opposite to all the all other planets. Venus has a very thick atmosphere, composed largely of sulphuric acid and CO2. The surface temperature of Venus has been recorded as high as 475oC, which is hotter than Mercury. This is because of the greehouse effect of Venusís atmosphere. The clouds within the atmosphere trap heat, raising the surface temperature. The surface of Venus is covered with craters, volcanoes, and large ridges which may be evidence of mountain building.

Mars has a diameter of 6790 km, just over half of the Earth. It takes Mars 687 days to revolve once around the Sun. Its orbit is elliptical, ranging from 206-249 million km in diameter. The surface of Mars has an intense reddish orange hue, which is caused by large amounts of iron oxide in rocks. Mars has a very thin atmosphere, which is composed largely of CO2. Its surface is very cold, and is covered with craters, volcanoes, and large canyons, which may indicate the past presence of water. Maximum surface temperatures range from -29oC to -100oC. Mars has two tiny satellites called Phobos and Deimos. Phobos is fast; it travels 3 times around Mars in one day!

  1. In this exercise, the students will construct a model of the orbits inner or terrestrial planets. First, go over some of the information on the terrestrial planets, making sure students are able to compare and contrast the different planets.
  2. Have the students use compasses to make the appropriate sized circles for each orbit. If you do not have compasses, you can either make circles that act as templates before the exercise or use two pencils tied together with a string measured at the appropriate distance.

    Use the following ratio for the orbits: Sun: Mercury: Venus: Earth: Mars = 0: 1: 2: 3 1/2: 4 1/2. For example, if you are using the metric system, the Sun would be 0 cm. Mercury's orbit would be 10 cm in diameter, Venus' orbit is 20 cm, Earth's orbit is 35 cm and Mars' orbit would be 45 cm. If your paper is not large enough for these dimensions, cut the diameter of each of the planets by half.

    Have the students cut out the orbits.

  3. Have the students push a brad through a sheet of paper. This will represent the Sun. Stack the orbits on the brad, starting with the largest orbit (Mars), then Earth, followed by Venus, and finally Mercury.
  4. Have the students revolve each disk in the correct direction, using the chart on the worksheet for guidance. All four planets should revolve counterclockwise.
  5. Remind the students that the orbits of the planets are not circular, but have elliptical shapes, as shown on the Orbits of the Planets picture (from the Pre Lab). Have the students compare this with the Solar System Placemat. It is a closer look at the planets.

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