The ecliptic and the wandering stars

The ecliptic is the path the Sun traces against the background stars over a year — really the plane of the Earth's orbit projected onto the sky. Because all the planets orbit in nearly the same plane, they are always found close to the ecliptic, passing through the constellations of the zodiac. The ecliptic crosses the celestial equator at two points: the First Point of Aries (where the Sun crosses heading north, at the March equinox) and the First Point of Libra (September equinox).

Five planets are easily visible to the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. To the Greeks they were planetes — "wanderers" — because, unlike the stars, they change position night after night.

• Inferior planets — Mercury and Venus. Their orbits lie inside the Earth's.
• Superior planets — Mars, Jupiter, Saturn (and beyond). Their orbits lie outside the Earth's.

Spot the difference: planets shine with a steady light (they show a disc) while stars twinkle. Venus is the brilliant "morning star" or "evening star" — never far from the Sun, so never high in the sky in the middle of the night.

Conjunction, opposition, elongation, transit

These five words describe where a planet sits relative to the Sun (or another body) as seen from the Earth:

• Conjunction — the planet appears close to the Sun in the sky (so it is lost in the glare). An inferior planet has an inferior conjunction (between us and the Sun) and a superior conjunction (behind the Sun).
• Opposition — the planet is opposite the Sun in the sky. Only a superior planet can do this. It is then closest to the Earth, at its brightest, and visible all night — the best time to observe it.
• Elongation — the angle between the planet and the Sun. An inferior planet has a maximum elongation (up to about 47° for Venus, up to about 28° for Mercury), which is why they never stray far from twilight.
• Transit — an inferior planet crossing the face of the Sun. Only Mercury and Venus can do it (from the Earth), and only at inferior conjunction when they are near a node.
• Occultation — one body passing in front of another and hiding it completely, e.g. the Moon occulting a star or a planet.

Phases: because they can lie between us and the Sun, inferior planets show a full set of phases — crescent, half, gibbous — exactly as Galileo found for Venus. A superior planet can only ever look full or slightly gibbous.

When to observe Mars

Retrograde motion

Normally a planet drifts slowly eastwards against the stars (prograde or direct motion). But every so often a superior planet slows, stops, loops backwards (westwards) and then resumes — this is retrograde motion.

It is an illusion caused by our own motion. The Earth, on a smaller and faster orbit, overtakes the superior planet on the inside — just as a fast car overtaking a slower one makes the slower car appear to slide backwards against the distant scenery. Retrograde motion therefore happens around opposition.

History matters: retrograde motion was the great puzzle for the geocentric model. Ptolemy needed complicated epicycles to reproduce it; in the heliocentric model it falls out automatically from the relative motion of the two planets.

Backwards loops

The synodic period

The sidereal period of a planet is one true orbit round the Sun. The synodic period is the time between one configuration (say, opposition) and the next identical one as seen from the moving Earth. They are different, and you can calculate one from the other:

Sense-check: for a very distant superior planet, P is huge, so 1/P ≈ 0 and S ≈ E = one year. Neptune reaches opposition every 367 days — barely more than a year.

Your turn — Mars at opposition

Your turn — how far can Venus get from the Sun?

Comets, meteors and meteor showers

Comets are "dirty snowballs" — a nucleus of ice and dust a few km across, on a long, highly eccentric orbit. As a comet nears the Sun the ice sublimes, producing:

• a coma (a glowing head of gas and dust), and
• two tails — a bluish ion/gas tail blown straight back by the solar wind, and a curved yellowish dust tail pushed by radiation pressure.

Meteors ("shooting stars") are tiny grains of dust burning up by friction about 80–100 km up. A meteor shower happens when the Earth ploughs through the dust trail left by a comet — the Perseids in August, from Comet Swift-Tuttle. All the meteors seem to stream out of one point, the radiant, because they are travelling on parallel paths (a perspective effect).

Vocabulary marks: meteoroid = the lump in space · meteor = the streak of light in the atmosphere · meteorite = what actually lands on the ground.

Your turn — meteor rates

Inferior or superior?

Tap a statement, then tap the kind of planet it applies to.

Match the description to the term

Tap a description on the left, then its matching term on the right.

Which way does the tail point?

Naming the rock

What a small telescope shows

You should know what is actually visible on each planet through a small telescope:

• Venus — a brilliant, featureless white disc showing clear phases (its surface is hidden under permanent cloud).
• Mars — a small orange disc; at a good opposition you may see dark markings and a white polar cap.
• Jupiter — a banded disc with dark belts and light zones, the Great Red Spot, and the four Galilean moons (Io, Europa, Ganymede, Callisto), which visibly change position from night to night.
• Saturn — the ring system (with the Cassini division) and its largest moon, Titan. The rings appear to open and close over Saturn's 29.5-year orbit because our viewing angle changes.
• Mercury — hard to catch: never more than about 28° from the Sun, so only visible low in twilight.

Observing the Sun: never look directly at the Sun, with or without a telescope or binoculars. Instead let sunlight pass through a small pinhole in a card and view the image it forms on a second card — pinhole projection is the safe method Edexcel expects. Aurorae and zodiacal light are also naked-eye phenomena worth knowing: the aurora comes from the solar wind hitting the upper atmosphere near the poles; the faint cone of zodiacal light before dawn or after dusk is sunlight scattered off dust in the plane of the solar system.

Four points of light

The big ideas to know

That is Solar System Observation covered for Edexcel GCSE Astronomy. Press Finish to see your score.