Just the Facts – Solar System by School Specialty Publishing – SSP0742419886

HISTORY OF THE SOLAR SYSTEM
The solar system probably began to form about 5,000 million years ago.

• A vast cloud of space gas and dust began to clump together under its own pull of gravity. The clump began to spin.
• The center of the clump became the Sun.
• Much smaller bits spinning around it became the planets and perhaps some moons.
• Most of the solar system, including Earth, was formed by 4,500 million years ago.
• The solar system is probably only about one-third as old as the universe itself.

OUR CHANGING VIEWS
Beliefs about the solar system and universe have constantly evolved.

• In ancient times, people thought all objects seen in the skies went around Earth.
• Gradually, scientific observations showed that Earth and other planets orbited the Sun.
• The invention of the telescope around 1609 confirmed this idea and allowed the discovery of many more space objects.
• From the 1930s, astronomers realized that some space objects gave out invisible radio waves, as well as or instead of light rays.
• Radio telescopes allowed discovery of yet more objects in space, many invisible to ordinary optical telescopes, because they give out no light.
• More kinds of rays were discovered coming from space objects.
• From 1990, the Hubble Space Telescope has discovered more stars and other space objects.

WRONG IMPRESSIONS
In solar system diagrams, it is almost impossible to get a true idea of distance and scale onto an ordinary page.

• The planets are tiny compared to the Sun. Even the biggest, Jupiter, would fit into the Sun more than 1,000 times.
• The four inner planets are relatively close to the Sun, but distances become ever greater with planets farther from the Sun.
• Diagrams must show the planets far bigger closer to the Sun and closer together than in real scale, just to fit them on a page.

ORBITS AND ECCENTRICITY
Most orbits, especially those of the planets around the Sun, are not exact circles.

• They are shaped more like ellipses or ovals.
• The Sun is not in the center of the oval of most orbits, but slightly offset toward one end, near one of the points called the focus.
• The amount that a planet’s orbit differs from a circle is called eccentricity.
• The bigger the eccentricity, the more elliptical the orbit.

Planet Eccentricity
Mercury 0.205
Venus 0.006
Earth 0.016
Mars 0.093
Jupiter 0.048
Saturn 0.054
Uranus 0.047
Neptune 0.008
Pluto 0.248

Venus has the most circular orbit, closely followed by Neptune, while Pluto’s is the most oval orbit, followed by Mercury.

SOME SPACE UNITS
Astronomical unit (AU) – The average distance from the Earth to the Sun is 93 million miles.

Light year (l-y) – The distance that light (which has the fastest and most constant movement in the universe) travels in one year, is 5.88 trillion miles.

Parsec (pc) – 19.2 trillion miles, defined by a star’s apparent shift in position (parallax) when viewed from two points which are a distance apart equal to the distance from the Earth to the Sun, that is, one AU.

Axial tilt – The angle at which the axis, the imaginary line around which a planet spins, is tilted compared to the level of the solar plane.