The celestial sphere

It is convenient to pretend the stars are stuck on the inside of a huge celestial sphere centred on the Earth. Project the Earth's features outwards and you get the sphere's landmarks:

• North and south celestial poles — straight above the Earth's poles. The sky appears to rotate about them (Polaris sits within 1° of the north celestial pole).
• Celestial equator — the projection of the Earth's equator.
• Zenith — the point directly overhead for you.
• Meridian — the imaginary line running from due north, through the zenith, to due south. An object is at its highest when it crosses the meridian (it culminates).
• Ecliptic — the Sun's yearly path, tilted 23.5° to the celestial equator.

The whole sky turns once every sidereal day (23 h 56 min) about the celestial poles — an illusion produced by the rotating Earth.

Two coordinate systems

There are two ways to say where an object is.

• Altitude & azimuth — the horizon system. Altitude = degrees above the horizon (0° horizon, 90° zenith). Azimuth = degrees measured clockwise from due north (N = 0°, E = 90°, S = 180°, W = 270°). It is easy to use — but the values change every minute, and differ for every observer.
• Right ascension & declination — the equatorial system, fixed to the stars themselves, like latitude and longitude on the sky. Declination (dec) = degrees north (+) or south (−) of the celestial equator. Right ascension (RA) = measured eastwards in hours, minutes and seconds (0 h to 24 h) from the vernal equinox.

Why RA is in hours: the sky turns 360° in 24 sidereal hours, so 1 hour of RA = 15°. A star with an RA 1 h greater than another crosses the meridian 1 sidereal hour later.

Which coordinate?

Circumpolar stars

As the sky rotates, some stars near the celestial pole never dip below the horizon — they are circumpolar and can be seen on any clear night of the year. From the UK, Ursa Major and Cassiopeia are circumpolar.

And the maximum altitude of a star as it crosses the meridian (for a star south of the zenith) is: altitude = 90° − latitude + declination. For Sirius (dec = −16.7°) from 52° N: 90 − 52 − 16.7 = 21.3° — it never gets high, which is why it flickers so dramatically.

Your turn — circumpolar limit

Your turn — how high does it climb?

The magnitude scale

Brightness is measured on the apparent magnitude scale, invented by Hipparchus and still deliberately backwards:

• A smaller (or more negative) number means a brighter object. Sirius is −1.4; Polaris is about +2.0; the faintest naked-eye stars are about +6.
• The scale is logarithmic: a difference of 5 magnitudes = exactly 100× in brightness. So one magnitude = 100^1/5 ≈ 2.512×.

Apparent magnitude tells you how bright it LOOKS, which depends on distance as well as on the star's true output. To compare stars fairly you need absolute magnitude — the magnitude a star would have at a standard distance of 10 parsecs.

Your turn — how much brighter?

Brighter or fainter?

Binoculars and telescopes

Beyond the naked eye, the key property of any instrument is its aperture — the diameter of its main lens or mirror.

• Light grasp — collecting area goes as (aperture)², so a bigger aperture shows fainter objects.
• Resolution — a bigger aperture separates finer detail.
• Magnification — set by the eyepiece, not by the aperture:

Binoculars are labelled like 10 × 50 — magnification ×10, aperture 50 mm.

Observing skills that earn marks: allow 20–30 minutes for dark adaptation; use a red torch so you keep it; use averted vision (look slightly to one side) for faint objects; observe when the object is high, away from horizon extinction and light pollution.

Which system does it belong to?

Tap a statement, then tap the system it belongs to.

Match the description to the term

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

Choosing an eyepiece

Seeing the faintest stars

Constellations, asterisms and star names

A constellation is one of the 88 official regions the whole sky is divided into (Ursa Major, Orion, Cassiopeia...). An asterism is just a recognisable pattern that is not itself a constellation — the Plough is an asterism within Ursa Major, and the Summer Triangle spans three constellations.

• The stars in a constellation usually have nothing to do with each other — they are at wildly different distances and only appear close together by chance alignment.
• Bright stars carry Greek letters in rough order of brightness within their constellation: α (alpha) Orionis is Betelgeuse, β (beta) Orionis is Rigel.
• Signposts worth knowing: the Plough's "pointers" lead to Polaris; Orion's belt points down-left to Sirius and up-right to Aldebaran.

Which constellations you see depends on the time of year — because at midnight you face away from the Sun, and that direction moves right round the ecliptic over 12 months. Orion is a winter constellation in the UK; in June it is up in daylight.

Pattern or region?

The big ideas to know

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