Topic 6 – Celestial observation — Edexcel GCSE Astronomy
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Topic 6 – Celestial observation explained
Asterisms act as signposts to locate specific objects.
Read the full explanation
From the Plough, follow the curve of the handle away from the bowl to reach Arcturus, and use the two stars at the end of the bowl (the Pointers) to point to Polaris. From Orion’s Belt, follow the line of the three Belt stars down and left to Sirius, and up and right to Aldebaran; extending this line further past Aldebaran leads to the Pleiades cluster. From the Square of Pegasus, extend a line southwards through the two stars on the western side to find Fomalhaut. To locate the Andromeda galaxy, start at the top-left star of the Square (Alpheratz) and follow the adjacent stars of Andromeda, or extend a diagonal line across the Square through Alpheratz. Learn each hop as a direction and distance.
Your focus
- Use the Plough to locate Arcturus and Polaris.
- Use Orion’s Belt to locate Sirius, Aldebaran and the Pleiades.
- Use the Square of Pegasus to locate Fomalhaut and the Andromeda galaxy.
Topic 6 – Celestial observation exam tips
Marking Points
- The Plough handle curve extended away from the bowl points to Arcturus, and the two stars at the end of the bowl point to Polaris.
- The line of Orion’s Belt extended downwards and to the left reaches Sirius.
- The line of Orion’s Belt extended upwards and to the right reaches Aldebaran, and continuing this line leads to the Pleiades.
- A line extended southwards through the two stars on the western side of the Square of Pegasus points to Fomalhaut.
- The Andromeda galaxy is located by extending a diagonal line across the Square of Pegasus through its top-left star (Alpheratz), or by star-hopping from Alpheratz.
Examiner Tips
- 💡Draw each pointer line on a star chart and note the direction and approximate distance to the target.
- 💡Learn the Belt as a signpost: down to Sirius, up through Aldebaran to the Pleiades.
- 💡Remember that the Square of Pegasus uses the western edge for Fomalhaut and a diagonal through the top-left star for the Andromeda galaxy.
Common Mistakes
- Using the Plough handle to find Polaris: the handle curve leads to Arcturus, while the two bowl-end stars point to Polaris.
- Extending Orion’s Belt in only one direction: the Belt line points to Sirius in one direction and Aldebaran in the other.
- Mislocating the Pleiades: you must extend the line of Orion's Belt upwards and to the right, passing through Aldebaran, to reach the Pleiades.
- Using the northern edge of the Square of Pegasus to find Andromeda. Correction: use a diagonal line through the top-left star (Alpheratz) or star-hop from Alpheratz to find the Andromeda galaxy.