Other objects Asteroids in orbit between Mars and Jupiter. Comets (in highly eccentric orbits). Meteors are cometary debris and are called ‘shooting-stars’. Meteorites are related to asteroids.
The Planets All the planets move in similar shaped orbits in the same direction (direct motion) and in the same plane apart from Pluto (which has an orbital inclination of 17º). This is the Zodiac. The interplanetary medium reflects sunlight to create Zodiacal Light . The planets formed out of a solar nebula . The inner planets lost most of their lighter matter whereas the outer larger planets retained hydrogen and helium. The age of the Earth is about 4600 million years .
Planetary Data 0.248 17º12’ 6d 9h 247.7 yrs 4425 7375 5900 Pluto 0.009 1º45’ 16h 3m 164.8 yrs 4456 4537 4497 Neptune 0.047 0º48’ 17h 14m 84 years 2735 3004 2870 Uranus 0.056 2º29’ 10h 38m 29.5 years 1347 1507 1427 Saturn 0.048 1º18’ 9h 50m 30s 11.9 years 741 816 778 Jupiter 0.093 1º51’ 24h 37m 23s 687 days 207 249 227.9 Mars 0.017 0 23h 56m 4s 365.3 days 147 152 149.6 Earth 0.007 3º24’ 243.2 days 227.7 days 107.4 109 108.2 Venus 0.206 7º 58.6 days 88 days 45.9 69.7 57.9 Mercury min max mean Orbital eccentricity Orbital inclination Rotation period Sidereal period Sun from the (10 6 km) Distance Planet
The Moon It is thought the Moon was created from the Earth; a giant rock carved a chunk of material out of the Earth and this became the Moon. The Moon reflects sunlight, however it has a low reflectivity, reflecting only 7% of the incident sunlight. We would need 465 000 full moons to receive the same amount of light as from the Sun! The Moon’s orbital period is 27.3 days but the time interval between successive new moons is 29.5 days . This is because the Earth has moved in its orbit around the Sun.
Phases of the Moon New Moon Waxing Crescent Waxing Gibbous First Quarter Waning Crescent Waning Gibbous Last Quarter Full Moon
Phases of Mercury and Venus Galileo Galilei (1564 - 1642) observed the phases of Venus Crescent Dichotomy Gibbous Full
Phases of Venus 1 Inferior conjunction 3 Superior conjunction 4 Eastern elongation 2 Western elongation
Phases of the inferior planets At position 1 (inferior conjunction) the planet is new and we see the dark side. Occasionally this dark spot can be seen moving across the Sun in the sky - this is a planetary transit. However the orbits are tilted so these transits are not common. Mercury was in transit in May 2003, Venus was last seen in transit in June 2004 (below).
Phases of the inferior planets At position 2 and 4 (elongation) the planet is at half-disc, this is termed dichotomy. At dichotomy the planet is at its greatest angular distance from the Sun (47º Venus, 28º Mercury). At position 3 (superior conjunction) the planet is behind the Sun and is above the horizon only during the daylight. It is at its greatest distance from Earth and its angular diameter (apparent size in the sky) is at a minimum. The superior planets (Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) never come to inferior conjunction as these are in larger orbits than the Earth.
Orbit of Mars When the Earth and Mars are in positions E1 and M1 the Sun and Mars are opposite in the sky. This is when Mars is at opposition and is best for observing; it is at its highest in the sky at midnight. E1 E2 M1 M3 M2
Orbit of Mars A year later, the Earth is back at E1 but Mars which has a smaller orbital velocity will only have reached position M2. Mars is back at opposition when Earth is at E2 and Mars is at M3. The mean interval between successive oppositions (the synodic period ) is 780 days . There were oppositions in 2005 and 2007, but not in 2006 or 2008. Jupiter has a synodic period of 399 days , so the planet is visible for several months each year. Saturn is at opposition every 378 days . Away from opposition Mars can show a gibbous phase . It can pass through a superior conjunction. Due to its slower orbital velocity relative to the Earth, Mars can appear to move backwards in the sky relative to the stellar background - it exhibits retrograde motion . This happens when the Earth passes Mars on the inside.
Orbit of Mars Mars’ orbit is more eccentric than the Earth’s so the opposition distance is not constant . When Mars is at perihelion the opposition distance (from Earth) is 58 million km, at aphelion the distance is 100 million km. Perihelion Aphelion
Comets Comets have highly eccentric orbits. Some move in retrograde orbits, e.g. Halley’s comet which has an orbital period of 76 years. It was last seen (at perihelion) in 1986, the next visible point is in 2061.