For the first time in more than 100
years, astronomers on Planet Earth were able to view a Transit
of Venus. On June 8, 2004, Venus appeared to travel across the
face of the sun. This exciting and rare astronomical event could
be seen by observers in most of the world.
Venus transits currently occur at
intervals of 8, 105.5, 8, and 121.5 years. Only six transits of
Venus have occurred since the invention of the telescope.
Click this button to learn more about
the Transit of Venus.
The Sun, nine planets, and their satellites, make up the Solar System, but only five of these bodies, Venus,
Mars, Jupiter, Saturn, and the moon, make ideal telescope targets. The Inner Planets, Mercury and Venus, are so
classified because they are inside the orbit of the earth. The remaining planets are classified as Outer Planets
because of their position outside the earth's orbit.
Planetary observing is one of the most accessible of observing
pursuits for amateur astronomers, now that
most of us live in light polluted urban centers. Because of their brightness, planets can be seen on any clear
night from inside the limits of any large city, even when the bright stars are not visible.
Planetary observing can be done either visually or with optical equipment, such as binoculars or a telescope.
With the unaided eye, it is easily possible for anyone to note the movements of the five brighter planets across
the sky from day to day and from month to month. With binoculars, it is possible to see the satellites of Jupiter
and locate the positions of Uranus and Neptune. Using a telescope, the interested observer can note the changing
positions of the satellites of Jupiter and Saturn, and make regular observations of changing features on the surface
of any of the bright planets.
Seeing conditions rather than telescope aperture are more important in planetary observing than in deep
sky work, due to the need to use higher powers at the eyepiece. If there is a high degree of atmospheric
images in the field of view will become distorted and blurry, making fine detail impossible to detect. For this
reason, it is best to try to avoid looking over the tops of high buildings or the roof of your house, which may
be radiating heat upward.
Use this page as a reference to basic data on the planets and their satellites as you
persue your observations.
Earn The Astronomical League's Award for Observing the Solar System
A. The Solar Nebula Theory: Our solar system was probably formed out of a spinning ball of gas. When
the sun became luminous enough, the remaining dust and gas were blown away into space, leaving the planets orbiting
the sun This happened about 4.5 billion years ago.
B. The planets move around the sun in orbits that lie nearly in a common plane, and they all revolve about the
sun in the same direction (counter-clockwise as seen from the North Pole).
C. By the process of condensation, the heavier elements condensed toward the hotter, inner parts of the nebula,
and the lighter, more volatile elements condensed further out. Therefore two distinct types of planets exist. The
Terrestrial (Earth-like) planets, Mercury, Venus, Earth and Mars, are small, dense and rocky. The
(Jupiter-like) outer planets, Jupiter, Saturn,. Uranus, and Neptune, are enormous gas giants, and are larger, less
dense, and much colder.
D. As the solar nebula cooled, small aggregates of material called planetesimals began to accrete mass.
The inner planetesimals accrued heavier material through condensation and the fact that the lighter material had
been swept away by the solar wind. The planetesimals further out accrued lighter material. As the planetesimals
grew larger, they began to act as vacuum cleaners, ridding the solar system of debris and eventually becoming the
E. The solar system is left with three types of debris from the solar nebula: asteroids, comets, and meteoroids.
1. Asteroids are small rocky worlds. Most asteroids orbit the sun in a region between Mars and Jupiter
called the Asteroid Belt. This belt is thought to be the remains of a planet that failed to form at a distance
of 2.8 AU from the sun. There are about 200 asteroids larger than 60 miles in diameter, about 200 larger than 6
miles, and 500,000 larger than 0.6 miles.
2. Comets are large dirty snowballs. They consist of three parts. The nucleus is a ball of frozen water
and carbon dioxide, and is usually a few dozen Km in diameter. The coma is the bright area surrounding the nucleus,
and may be as large as Neptune. The tail is a luminous trail of debris left behind as the comet melts. As a comet
falls into the sun from the Oort Cloud or the Kuiper Belt, it steadily becomes warmer. At about the
same distance from the sun as Jupiter (about 5 AU), it begins to melt. The debris it leaves behind behaves similarly
to exhaust from a car on a windy day. If there is no wind, the exhaust trails out behind the car. But if it is
very windy, the exhaust will go in the direction of the wind, no matter how fast the car is going. A comet travels
at about 30 Km/s. The Solar wind, which is continuously emitted from the sun, gusts from 300 to 800 Km/s.
The tail of the comet is therefore always blown away from the sun by the solar wind, which also causes it to be
3. A meteorite is a space object (a piece of debris. a pebble, or a grain of sand) that survives its
plunge through the atmosphere to hit the Earth. Before it hits the Earth, it is called a
meteoroid. A meteor
is the streak of light across the night sky that one sees as the object burns up in the atmosphere.
are events that feature many meteors impinging on the Earth at one time. The usual cause for these showers are
events that feature many meteors impinging on the Earth at one time. The usual cause for these showers are the
trails of debris left by comets. The showers tend to occur at the same time every year, and have their greatest
concentration at the same point in the sky. They are therefore often designated by a name referring to the constellation
in which they appear to emanate. For instance, the meteor shower Taurids has its greatest concentration of meteors
in the constellation Taurus between November 1-7, and is caused by the Comet Encke.
The Leonid Meteor Showe between November 17-19 is caused when the Earth
passes throught debris left from Comet Swift-Tuttle.
Planetary Orbital Data
Distance from Sun
Millions of Miles
Distance from Earth
Distance from Earth
Period of Revolution
Period of Revolution
Diameter in Miles
Millions of KM
SIDERIAL Period: The time for a planet to make one revolution around the sun. It is the planet's "year"
in terms of earth days.
SYNODIC Period: The time between successive similar aspects of a planet in the sky. This is equal to one "lap"
of the earth as both planets revolve around the sun.