In 2006 Members of the International Astronomical Union decided Pluto should be stripped of its planet status and classified as a dwarf planet along with charon in the asteroid belt and UB313, Sedna was still not classified as of Aug., 2006.
See the Planet Page for more information.

The Solar System is made up of the sun, the planets (major, planetoid, dwarf planets, plutoid), asteroids and comets. In addition there are man made objects, the spacestation and satellites.

Solar system distances are usually given in terms of Astronomical Units (AU), the average distance from earth to sun = 92.96 Million (M) mi = 149.60 M km
The solar system is made up of:

  • 0.5-3 AU - The inner planets: Mercury, Venus, Earth, Mars.
  • 5-7 AU - Asteroid Belt
  • 10-60 AU - The outer planets - Jupiter thru Neptune
  • 60-100 AU - The Kuiper Belt (large comets)
  • 100-200 AU - The Oort Cloud - A spherical region, as opposed to a disk shape for the objects above, with comets -
  • 240 AU - Heliopause - The region of space where the sun's solar wind meets that of other stars.
(Note: above distances are from edge to edge; distance from the sun is 1/2 that.)

Most of the planets orbit in the same plane called the eliptic plane.
The ecliptic and defined by the plane of the Earth's orbit. The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. It is inclined by 23.5° from the earth's equator since the Earth's spin axis is tilted 23.5° with respect to its orbit around the sun.
The sun, planets and moon travel close to this plane. Mercury's orbit is inclined at 7.0° from the ecliptic.
Venus's orbit is inclined by 3°
Pluto's orbit is inclined at 17.1°
Sedena's orbit is inclined at 11.9°

The plane of the planets is perpindicular to the plane of the gallaxy.

All the planets rotate in the same direction as their orbits (counter clockwise when looking from the north) except Venus, and Uranus and Pluto. Venus rotates clockwise when viewed from the north the sun rises in the west. Uranus' and pluto's axis are almost parallel to the ecliptic (i.e. tilted 90°).

None of the planets have a perfectly circular orbit (eccentricity = 0). Most have slightly oval orbits (eccentricity ranges from .007 - .09). The orbits of Mercury (.21), Pluto (.25) and Sedna (.84) are more oval (eccentricity > .20). See eccentricity below.

A planet will be moving at its greatest speed at perihelion. At aphelion it will be moving at its slowest. The earth is moving at 68,893 MPH (30.80 km/s) at perihelion and 64,442 MPH at aphelion.

The Sun orbits around the center of the Milky Way Galaxy at a speed that is about 230 kilometers per second. This is in a frame centered on the Galaxy and which does not rotate. Most of the stars near the Sun have similar motions.

The Galaxy itself is moving at several hundred kilometers per second with respect to a reference frame defined by the cosmic microwave background (radiation left over from the Big Bang).

Other bodies:

There are 26 known asteroids larger than 200 km in diameter.
Ceres is the largest by far at 933 km in diameter. Most are in the asteroid belt, but some are closer in or farther out.

Chiron was discovered in 1977. It lies between Saturn and Uranus and occasionally passes within the orbit of Saturn. At first thought to be a comet or asteroid it is now considered a "planetoid" or small planet-like body. It is 170 km in diameter.

Our Sun, with its family of planets, is racing around the center of our Galaxy (moving towards the constellation cignus, north of Capricorn) at about 250 km/sec (563,000 miles per hour). Even at that incredible speed, our solar system requires about 225 million years to complete just one revolution.

4.4 billion years ago: The Moon forms as a satellite of Earth

Constellations are groups of prominent stars in an area of the sky that can be perceived as a geometrical form or a picture. Each culture has had its ow way of dividing the sky into Constellations. Our system was originated in ancient Mesopotamia and elaborated by the Greeks. There are a total of 88 constellations.
We traditionally think of the year divided into 12 astrological constellations along the ecliptic. There are actually thirteen astronomical constellations that cross the ecliptic.
The zodiac is defined by the constellations that come within 8° or 9° of the ecliptic. There are 24 of these. Sign Astronomical Dates
Zodiacal and Ecliptic Constellations
Symbol Season Dates - sun enters and leaves # of
Aries Ram Dec 19-Apr 13-May 25 Mar 21 - Apr 19
Taurus Bull Jan 14-May 19-Jun 37 Apr 20 - May 20
Gemini Twins Feb 20-Jun 20-Jul 31 May 21 - Jun 20
Cancer Crab Mar 21-Jul 9-Aug 20 Jun 21 - Jul 22
Leo Lion Apr 10-Aug 15-Sep 37 Jul 23 - Aug 22
Virgo Maiden May 16-Sep 30-Oct 45 Aug 23 - Sep 22
Libra Balance June 31-Oct 22-Nov 23 Sep 23 - Oct 22
Scorpius Scorpion July 23-Nov 29-Nov 7 Oct 23 - Nov 21
serpent bearer July 30-Nov 17-Dec 18  
Sagittarius Archer Aug 18-Dec 18-Jan 32 Nov 22 - Dec 21
Capricornus Sea Goat Sep 19-Jan 15-Feb 28 Dec 22 - Jan 19
Aquarius Water Bearer Oct 16-Feb 11-Mar 24 Jan 20 - Feb 18
Pisces Fish Nov 12-Mar 18-Apr 38 Feb 19 - Mar 20
Season = Evening Culmination - Time of year when constellation is overhead at 9-10 PM
Astronomical and Astrological dates are when the sun enteres the constellation, so it cannot be seen in the night time sky then.
Astrological Dates are for the Tropical zodiac there is also a Sidereal zodiac and the dates change by year.

Serpens, The Serpent, and Ophiuchus, The Serpent Holder, originally formed one constellation. Serpens is now a two section constellation: Divided into head and body by the intervening Ophiuchus. They are crossed by the ecliptic but is not a zodiacal constellation.

Despite the fact that many diagrams represent the solar system as having each orbit the same distance apart, in actuality the orbits are largely arranged geometrically, that is, each is from 1 1/2 to 3 times the distance from the Sun as the one before it.
A close approximation of distances is given by Bode's Law.
Distance from sun in AU = (4 + 3x2n) ÷ 10
n = 0-Venus, 1-earth, 2-mars, 3-asteroid belt, 4-jupiter
(This pretty close except for neptune and pluto)

Real Constellations
Solar System and sky map software and web sites:
Sun signs by Sidereal zodiac and Tropical zodiac dates
See: Sky Map Web sites
Planetarium Software
Planet Orbital Positions

Places > Universe

The images of Jupiter, Saturn, Uranus and Neptune on the planets page and below were taken by the Voyager spacecraft from 1979 to 1989. The twin spacecraft Voyager 1 and Voyager 2 were launched by NASA at different times in the summer of 1977. Originally planned to last 5 years to investigate Jupiter and Saturn, the spacecraft continued to function and were directed to flyby Uranus and Neptune.

The Voyager mission was designed to take advantage of a rare geometric arrangement of the outer planets in the late 1970s and the 1980s which allowed for a four-planet tour for a minimum of propellant and trip time.

Voyager 1 is now leaving the solar system, rising above the ecliptic plane at an angle of about 35 degrees at a rate of about 520 million kilometers (about 320 million miles) a year. Voyager 2 is also headed out of the solar system, diving below the ecliptic plane at an angle of about 48 degrees and a rate of about 470 million kilometers (about 290 million miles) a year.
As of January, 2006 Voyager 1 was 98 AU (15 B km) from the sun and Voyager 2 was 79 AU (12 B km) from the sun. (For reference pluto's average distance is 6 B km)

Both spacecraft will continue to study ultraviolet sources among the stars, and the fields and particles instruments aboard the Voyagers will continue to search for the boundary between the Sun's influence and interstellar space. The Voyagers are expected to return valuable data for two or three more decades. Communications will be maintained until the Voyagers' nuclear power sources can no longer supply enough electrical energy to power critical subsystems.

Source: Voyager at NASA.

None of the planets orbits are a perfect circle; Most are close to a circle (eccentricity (e) < .1), but Mercury (e=.21), Pluto (e=.25) and Sedna (e=.84) are more elliptical .
The sun will be at one of the focal points in an elliptical orbit; The orbit perihelion is the point when the planet is closest to sun and aphelion is the farthest point. Eccentricity is a meassure of how elongated the orbit is. A circle has an eccentricity of 0 and a parabola (objects like a few comets which go out into space and never come back) have an eccentricity of 1.

See gravity
, Table of Planets, Earth at Perihelion, Eccentricity graph.
The Earth's orbital eccentricity changes with a period of about 100,000 years in the range of .005 to .06.

See Also:
Planet Orbital Positions at
Video, by Robert Hurt at the Spitzer Science Center at CalTech, showing a zoom out from the earth to Sedna to the Oort cloud
Planets to Scale
Constellations at
Our Solar System
Formation And Evolution Of The Solar System at Univ. of Cincinnati
The 12 Manifestations Map of the Evolutionary Ages Zodiac and Earth, Air, Water and Fire
Astronomical Calendar ($25) and Astronomical Companion ($18), by Guy Ottewell of Furman University.
Glossary at Astronomical Evengs
Night Sky Glossary
Orbit Diagrams at NASA
Interactive Solar System at NASA

last updated 23 Apr 2006