Why do different places keep different clock times, and how did sailors find their longitude?
Local time and longitude, time zones, GMT and UT, and the astronomical and horological methods of determining longitude, including Harrison's marine chronometer.
A focused answer to Edexcel GCSE Astronomy statements 4.15 to 4.21, covering how local time depends on longitude, the use of time zones, GMT and Universal Time, and the astronomical (lunar distance) and horological (Harrison's marine chronometer) methods of determining longitude.
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What this dot point is asking
Edexcel statements 4.15 to 4.21 want you to understand how local time differs between observers at different longitudes, the use of time zones, that mean time on the Prime Meridian is GMT (the same as Universal Time, UT), and how longitude is determined by astronomical methods (the lunar distance method) and the horological method (Harrison's marine chronometer).
Local time and longitude
This conversion ( degrees per hour, or minutes per degree) is the heart of every longitude and time-zone calculation. If you know two places differ by a given longitude, multiply by minutes (or divide by for hours) to get the time difference. The eastward rotation means the more easterly observer always has the later clock reading for the same instant of local solar time.
Time zones, GMT and UT
Without time zones, every town a few miles east or west would keep a slightly different clock, which is impractical, so each zone adopts the mean time of a central meridian. GMT/UT is the global reference from which other zones are offset (for example UTC+1, UTC-5). The Prime Meridian through Greenwich is the agreed zero of longitude and of civil time.
Finding longitude: the astronomical method
The Moon moves about its own diameter against the stars each hour, so its position works as a slow celestial clock. By measuring the lunar distance and looking up what Greenwich time gives that configuration, a sailor without an accurate clock could still recover Greenwich time and hence longitude. It was difficult and error-prone, which is why an accurate timekeeper was so valuable.
Finding longitude: the horological method
This was the practical solution to the longitude problem: carry Greenwich time with you. If local noon occurs when the chronometer reads 15:00 GMT, the ship is 3 hours, or degrees, west of Greenwich. You are not required to know how the chronometer works internally, only the principle: keep a reference time, find local time, convert the difference to longitude.
How Edexcel examines this
This is naked-eye Paper 1 content with steady calculation and explanation marks. Time and longitude calculations use degrees per hour (or minutes per degree): convert a longitude difference to a time difference, or a time difference to longitude, and state which place sees the Sun first (the more easterly). GMT and UT are tested by recall: mean time on the Prime Meridian, with GMT equal to UT. The longitude methods are explained: the lunar distance method (the Moon as a celestial clock against the stars, compared with Greenwich-referenced tables) and the horological method (Harrison's chronometer keeping Greenwich time, compared with local time from the Sun). A frequent synoptic link is to the Equation of Time, since a real sundial must be corrected before its local time is used. The biggest errors are dropping the degrees per hour factor and reversing east and west, so lock those in.
Try this
Q1. State the time difference for each degrees of longitude. [1 mark]
- Cue. One hour ( degree = minutes).
Q2. State what GMT is defined as, and the equivalent international time. [1 mark]
- Cue. Mean Solar Time on the Prime Meridian; the same as Universal Time (UT).
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 20223 marksTwo places are separated by 45 degrees of longitude. Calculate the difference in their local solar time, and state which place sees the Sun rise first.Show worked answer →
The Earth rotates degrees in hours, which is degrees per hour, so each degree of longitude is minutes of time (1 mark). For degrees the time difference is (or ) (1 mark). The place further east sees the Sun rise first, because the Earth rotates from west to east, so the Sun reaches eastern longitudes earlier (1 mark). Markers reward the conversion ( degrees per hour or minutes per degree), the answer of 3 hours, and identifying the more easterly place as seeing sunrise first.
Edexcel 1AS0 20214 marksExplain how a marine chronometer set to Greenwich time allows a navigator to find their longitude at sea (the horological method).Show worked answer →
The chronometer keeps the time at Greenwich (GMT) accurately throughout the voyage (1 mark). The navigator finds their own local time at sea by observing the Sun, for example noting when the Sun is highest (local noon) (1 mark). The difference between local time and the Greenwich time on the chronometer gives the difference in longitude, since hour of time difference equals degrees of longitude (1 mark). For example, if local noon occurs when the chronometer reads 15:00 GMT, the ship is hours, or degrees, west of Greenwich (1 mark). Markers reward keeping Greenwich time on the chronometer, finding local time from the Sun, and converting the time difference to longitude at degrees per hour. Knowledge of the internal workings of the chronometer is not required.
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