How do astronomers give a fixed address to a star, and a local position in our sky?
The celestial sphere, poles and equator, the equatorial coordinate system (right ascension and declination), the horizon coordinate system (altitude and azimuth), and hour angle and local sidereal time.
A focused answer to Edexcel GCSE Astronomy statements 6.7 to 6.12, covering the celestial sphere, poles and equator, the equatorial coordinate system (right ascension and declination), the horizon coordinate system (altitude and azimuth), and how an observer's latitude and meridian link them through hour angle and local sidereal time.
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What this dot point is asking
Edexcel statements 6.7 to 6.12 want you to understand the celestial sphere, poles and equator, the equatorial coordinate system (right ascension and declination), the horizon coordinate system (altitude and azimuth), and how the observer's latitude and meridian link the two systems through hour angle and local sidereal time, so you can choose the best time or object to observe.
The celestial sphere
Treating the sky as a sphere lets us assign coordinates to objects just as latitude and longitude do on Earth. The whole sphere seems to turn east to west each day, carrying the stars in circles, but this is the reflection of the Earth's eastward spin. The poles and equator of the celestial sphere are the natural reference framework for the equatorial coordinate system.
The equatorial coordinate system
RA and dec are the celestial equivalents of longitude and latitude. The First Point of Aries (the March equinox point, Topic 5) is the agreed zero of right ascension. A star catalogue lists RA and dec because they do not depend on where or when you observe, which makes them ideal for pointing a telescope or sharing a position.
The horizon coordinate system
Altitude and azimuth are what you actually use to point at something right now: " degrees up, in the south-east". But because the sky turns, an object's altitude and azimuth change minute by minute and differ for observers at different places, so these coordinates are local and time-dependent, unlike RA and dec.
Linking the systems: hour angle and sidereal time
This is how you convert between a star's catalogue position (RA, dec) and where it actually is in your sky (altitude, azimuth) at a given moment. Local sidereal time tells you which RA is currently on the meridian; combined with a star's RA you get its hour angle, and with your latitude you can work out its altitude and azimuth, and hence whether and when it is best placed to observe (statement 6.12).
How Edexcel examines this
This is naked-eye Paper 1 content with definition, comparison and short-calculation marks. The core question contrasts the two systems: equatorial (RA in hours from the First Point of Aries, dec in degrees, fixed to the sky) versus horizon (altitude above the horizon, azimuth around from north, changing with time and place). Hour angle and local sidereal time are tested by definition and by the relationship , sometimes as a small calculation. You may be asked to use coordinates and latitude to pick the best time to observe an object, or the best object for a time (statement 6.12). A reliable synoptic link is to the First Point of Aries (Topic 5) as the RA zero and to circumpolarity (next dot point). The commonest error is swapping the coordinate pairs, so anchor RA and dec to the fixed sky and altitude and azimuth to the local horizon.
Try this
Q1. State the two coordinates of the equatorial system and what each is measured from. [1 mark]
- Cue. Right ascension (eastwards from the First Point of Aries) and declination (north or south of the celestial equator).
Q2. State what altitude measures in the horizon coordinate system. [1 mark]
- Cue. The angle of an object above the horizon.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 1AS0 20214 marksExplain the difference between the equatorial coordinate system and the horizon coordinate system, including the two coordinates used in each.Show worked answer →
The equatorial coordinate system gives a fixed position on the celestial sphere using right ascension (measured eastwards along the celestial equator from the First Point of Aries, in hours, minutes and seconds) and declination (measured north or south of the celestial equator, in degrees) (2 marks). It is fixed to the stars, so a star's right ascension and declination do not change as the Earth rotates. The horizon coordinate system gives an object's position for a particular observer using altitude (the angle above the horizon) and azimuth (the angle measured around from north), and these change continuously as the Earth rotates and depend on the observer's location and time (2 marks). Markers reward the two equatorial coordinates (right ascension and declination, fixed to the sky) and the two horizon coordinates (altitude and azimuth, changing with time and place).
Edexcel 1AS0 20223 marksExplain what is meant by the hour angle of a star and how it relates to local sidereal time.Show worked answer →
The hour angle of a star is the angle, measured westwards along the celestial equator, from the observer's meridian to the star, telling you how long since (or until) the star crossed the meridian (1 mark). It increases steadily as the Earth rotates, so it acts as a measure of time since the star's upper transit (1 mark). The local sidereal time equals the right ascension of a star currently on the observer's meridian, and in general local sidereal time = hour angle + right ascension of the object (1 mark). Markers reward defining hour angle as the westward angle from the meridian to the star and linking it to local sidereal time via the relationship with right ascension.
Related dot points
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