Solar System

The information contained within the datasource is mostly from a single source, the excellent book 'Wanderers in space' by K. R. Lang and C. A. Whitney. Additional information was obtained from Views of the Solar System. I have made every effort to ensure that the information is accurate - but obviously I can't make any promises.

For any scientists out there, please note that at times I have sacrificed scientific accuracy for clarity when naming quantities on these pages. Why use 'Semi-major axis' when 'Distance from Sun' will do? I hope the entries below further clarify the meanings for all concerned:

Radius at equator (km)

The distance from the centre to the equator of the object in units of kilometres. For planets, this is usually slightly different to the distance from the centre to the poles. This is because as planets spin they are distorted slightly by the centrifugal force and squashed into an egg-like shape. The effect is strongest if the planet is spinning fast, and if it is gaseous rather than solid rock.

Radius (Earth = 1)

The radius of the planet in comparison to the radius of the Earth. This sort of comparison makes it easier to visualise the relative sizes of the planets.

Mass (x1022kg)

The mass of the object scaled by a factor of 1022 in units of kilograms. So if a planet has a mass of 1.2 x 1022 kg, it would weigh 12,000,000,000,000,000,000,000 kg! In astronomy we often have to resort to scientific notation like this - otherwise we would go giddy trying to keep track of all those zeros!

Average density (g/cm3)

A measure of how dense the object would be if its mass were evenly spread throughout its volume. In reality, planets and moons get denser towards their centres because the pressure of all the outer matter pushing down compacts the core material. The units are grams per centimetre cubed. For comparison, water has a density of 1 g/cm3. If the planet has a density less than one it would float, if it is greater than one it would sink.

Rotation period

The time taken for a planet to make one complete rotation around its spin axis. It is possible to measure the rotation period accurately (even for planets without a visible fixed surface features like Venus) by bouncing radio waves off the planet and detecting small shifts in frequency of the echo. The Earth's precise rotation period is known as the 'Sidereal Day' is actually around 4 minutes shorter than the mean solar day. This is because the Earth moves around its orbit by almost a degree over the course of a day, and the Sun's apparent position is shifted accordingly.

Temperature (�C)

The temperature of the planet in degrees Centigrade. Variations in these values are strongly dependent on the amount of atmosphere the planets have. Mercury's temperature varies between maximum and minimum over the course of a single revolution, because it has no atmosphere to absorb the energy of the Sun and redistribute it around the planet. The variations in temperature for Earth and Mars are on an annual rather than daily basis, and they arise because the orbits of these planets are not perfectly circular.

Escape velocity (km/s)

The speed an object would need to be travelling in order to be able to escape the grip of the planet's gravity, in kilometres per second. This value is dependent on the mass and radius of the planet.

Tilt of rotation (degrees)

The difference in angle between the planet's orbital plane and its spin axis. In other words how much the planet is tilted as it goes round the Sun. It is this tilt that causes the seasons on Earth and Mars, because each hemisphere is tilted towards the Sun half of the year, and away the other half. A value between 90 and 180 degrees indicates backward (retrograde) motion.

Distance from Sun (A.U.)

The greatest distance between the planet and the Sun in Astronomical Units (A.U.). One Astronomical Unit is defined as the distance between the Earth and the Sun, so this figure tells you how much closer or further away the other planets are relative to Earth.

Distance from Sun (millions of km)

Also known as the semi-major axis, this value is the furthest that the planet gets from the Sun in units of millions of kilometres. Because most of the planets travel round the Sun on nearly circular orbits it is generally possible to use this value as an estimate of the distance to the Sun.

Length of year (Earth days)

The time in Earth days it takes the planet to make one complete orbit around the Sun. This is known as a sidereal year and is measured relative to the background stars. This is important because planets also have a synodic year, which is the time it takes for the planet to return to the same position in the sky relative to the Sun from Earth. These two times are different because the Earth is also moving round the Sun, and our motion shifts the apparent position of the planets.

Average orbital velocity (km/s)

The average speed the planet is moving at as it travels around the Sun. Somewhat surprisingly this value changes over the course of a year, because the planets orbits are slightly elliptical. When the planets are near to the Sun they move slightly faster, due to a kind of slingshot effect.

Eccentricity

Eccentricity is a measure of how distorted the orbit of a body is from being a circle, with a value of 0 indicating a perfect circle. In practice most orbits have a small amount of ellipticity.

Orbital inclination (degrees)

The angle between the orbital plane of a body and some other reference plane. For the case of the planets, this is measured relative to the Earth's orbit. For the case of moons the reference is usually the orbital plane of the parent planet.

Satellites:

The number of known satellites orbiting the planet. Note that because moons shine only reflected light, it is difficult to see small, dark moons far from the Sun. This means we cannot be sure that we have detected all of the solar system's smaller moons. Data is only included for the principal satellites of the outer gas giants.

Parent planet

The planet around which the moon orbits.

Reflectivity:

Also called albedo, this figure represents the percentage of light that the moon reflects. A perfectly white object would have a reflectivity of 100% while a completely black body would have a value of 0%. The figure given is the average over the entire surface.

Distance from planet (km)

The average distance between the moon and its parent planet in units of kilometres. Although it seems that this value is constant for our Moon, it is infact moving away from us at a rate of around 4 centimetres per year. This is because orbital energy is being used up by the tidal interactions between the Earth and the Moon.

Distance from planet (RP=1)

The distance between the moon and its parent planet, scaled so that the planet has a radius of 1. So if the value is 40, it means that the moon is 40 times further away than the radius of the planet it is orbiting. This makes it slightly easier to visualise the relationship between the planet and its moons.

Orbital period (Earth days)

The time it takes in units of Earth days for the moon to make one complete orbit around its parent planet. The values for the moons of Mars show us that they are in dangerously low orbits, and will eventually crash into the Martian surface.