Universe > galaxy > solar system
Three scales, largest first: Universe — billions of galaxies (all matter, energy, space and time). Galaxy — billions of stars held together by gravity; ours is the Milky Way (~100 000 light-years across; 1 light-year m). Solar system — the Sun plus its 8 planets, their moons, asteroids and comets; distances in AU (1 AU m).
Gravity causes all orbits
Gravitational attraction causes every orbit: moons round planets, planets and comets round the Sun, satellites round Earth. Orbits are ellipses with the central body at one focus (a circle is the special case). Orbital speed ( in m/s, in m, in s). Planets orbit faster when closer to the Sun (smaller , shorter ); a comet is fastest at perihelion (nearest) and slowest at aphelion (farthest).
Comet tail points away from the Sun
A comet's tail always points directly away from the Sun, pushed out by solar radiation pressure and the solar wind — NOT trailing behind the comet. Structure: nucleus (ice and rock), coma (gas/dust cloud near the Sun) and tail. Comet orbits are highly elliptical, so they spend most time far from the Sun at low speed and are only visible periodically (e.g. Halley's Comet ~75 years).
Drawn from real examiner reports.
Universe > galaxy > solar system
Keep the hierarchy straight: Universe = billions of galaxies; galaxy = billions of stars; solar system = one star (the Sun) with its planets. Wrong forms: "the universe is billions of stars" (that describes a galaxy) and "our solar system is a galaxy" (a galaxy holds billions of stars). Name galaxies for the universe and stars for a galaxy.
June 2024 Paper 2P Q1 — candidates who placed non-stellar objects on the H-R diagram showed a confused distinction between a star and a solar system.
Comet tail points away from the Sun
The tail does NOT trail behind the comet like exhaust. Solar radiation pressure and the solar wind push it directly away from the Sun at every point in the orbit. So after perihelion, when the comet heads away from the Sun, the tail is in FRONT of the nucleus. A diagram with the tail drawn trailing behind scores zero.
Comet orbits are highly elliptical
Do not call a comet's orbit "elliptical, like a planet's" — that omits the key point. Planet and moon orbits are nearly circular (low eccentricity, small speed change). Comet orbits are highly elliptical (very elongated), with large speed variation: very fast near the Sun, very slow far away. State the degree of elongation, not just "elliptical".
Sun sits at a focus, not the centre
In an orbit diagram the Sun goes at one focus of the ellipse (Kepler's first law), not at the geometric centre. Drawing the Sun in the middle is penalised. An ellipse has two foci; a circle is the special case where the two foci coincide.
Don't drop the 2πr in v = 2πr/T
For orbital speed use the full circumference: . Computing (forgetting the ) is a common slip that scores zero. Also use the radius, not the diameter, and give the answer in m/s.
(general exam technique)
Closer planets orbit faster
A planet closer to the Sun moves faster, not slower — do not confuse a smaller orbit with a lower speed. Stronger gravity nearer the Sun gives a higher orbital speed and a shorter period. Comets show the same effect: they are fastest at their closest approach (perihelion).
(general exam technique)
Compare orbits by feature
For "differences in the orbits of comets, moons and planets", contrast by feature: shape (nearly circular vs highly elliptical), speed variation (small vs large) and period. Aim for one clear contrast per mark, and cover every orbit type named in the question.
Orbital speed: formula, sub, unit
State , substitute in SI units, evaluate, and quote the unit (m/s). To find or instead, rearrange first. Keep the and use the orbital radius, not the diameter.
Draw the tail along the Sun line
On a comet diagram, draw a line from the Sun to the comet and extend it outward — the tail lies along that line, away from the Sun, at every position. Also put the Sun at a focus and mark perihelion (fast) and aphelion (slow).
Keep powers of ten explicit
In scale calculations write the power of ten every time to avoid slips: 1 AU m; 1 light-year m. To convert light-years to metres, multiply by that value and check the exponent.
This topic is primarily qualitative, but one formula is tested (spec code 8.6):
where = orbital speed (m/s), = orbital radius (m), = orbital period (s).
Symbol definitions: ; is measured from the centre of the orbited body to the orbiting object; is the time for one complete orbit.
Scale units: distances within the solar system are quoted in astronomical units (AU): m. Distances to other stars and galaxies are quoted in light-years: m.
Define a galaxy.
The distance from the Earth to the Sun (1 AU) is m. The nearest star to our solar system (Proxima Centauri) is 4.24 light-years away. One light-year m.
Calculate the distance to Proxima Centauri in metres. (2 marks)
