History of Astronomy Example Exam Question Hints and Suggestions

Theme 3: Early Astronomy: Earth and Sky

First, note that this question consists of lots of short more-or-less independent sections. This means

  1. it would be easy to waste a lot of time. Keep answers concise and focused. On the other hand,
  2. it is possible to get quite a good mark on this question even if you can't do every part. For example, if you've forgotten what the saros is, it's only going to cost you 1.5 marks.
Briefly explain the difficulties facing ancient agricultural societies when defining a formal administrative calendar. [2]
Note that all you're being asked to do here is list the problems - not explain the solutions.

The basic argument is:

  • The essential period for agriculture is the solar year.
  • This is too long to expect a non-literate population to keep track of it directly, so you need to subdivide it.
  • However, the obvious smaller unit - the synodic lunar month, which is very easy to keep track of using the lunar phase - is far from being an integral subdivision of the solar year.
  • Therefore, simply counting lunar months will not provide a usable calendar for agriculture.
It would be good, but is not essential, if you could put numbers into this argument (e.g. synodic month ~ 29.5 days, so 12 synodic months ~ 354 days - lunar calendar will get a whole season (90 days) out of step in only 8 years).
Distinguish between lunar, solar and lunisolar calendars. [1.5]
Note the wording here: you need to focus on the differences between the three systems.
  • Lunar calendar: counts lunar months, not synchronised with solar year.
  • Solar calendar: based on solar year, does not keep track of lunar phase.
  • Lunisolar calendar: counts lunar months, but uses intercalation of extra months to maintain (on average) synchronisation with solar year.
Explain, giving examples, the astronomical methods that can be used to regulate lunisolar calendars. [3]
This section gets the most marks, so you need to give it the most time and space. Note that you are asked for astronomical methods, so you won't get any marks for "royal decree". You are also asked for methods, plural, so you need more than one, and for examples, so you need to provide some.

From the notes, the two astronomical methods are:

  • "observational: use a solar-year marker, e.g. the heliacal rising of a star, and apply a rule of the form "if this event occurs close to the beginning of the appropriate lunar month, the year has 12 months; if it occurs near the end, it has 13 months";
  • systematic: determine the true lengths of the lunar month and the solar year as accurately as possible, and find a good common multiple – then use this to design a pattern of intercalations."

The examples mentioned in the notes are the heliacal rising of Sirius (used to regulate the Egyptian lunisolar calendar) and the Metonic cycle (used by the Babylonians and the Chinese). As this is a 3-mark question, you probably need to go into more detail here, or to describe an additional example (the obvious one is the Indian calendar, regulated directly by the position of the Sun).

What is the saros cycle? Explain the relevance of the saros to the regulation of lunisolar calendars. [1.5]
Be sure to answer both parts of this question:
  • What is the saros?
    The saros is an eclipse cycle. From the notes, eclipse cycles are "common multiples of the synodic month and half the nodal month ... The most commonly used cycle in ancient times was the saros of 223 synodic months (= 242 nodal months)."
  • How is it used in calendar regulation?
    Again, from the notes, "eclipses have the advantage that the central moment of the eclipse fixes precisely the instant of Full Moon (lunar eclipse) ... Recording the precise times of eclipses in successive saros cycles therefore allowed ancient astronomers to refine their value of the length of the month."
In other words, using the saros helps astronomers to define the synodic month more precisely, which is useful if you are using a calculated cyclic rule to regulate your calendar.
Explain carefully any one of the following:
  • the origins of our own (Gregorian) calendar;
  • the Islamic calendar;
  • the Persian solar calendar.
[2]
Note that you are only asked to do one of these. It makes sense to choose the easiest, which for most people would be the Islamic calendar. Note also the difference in wording: for the Islamic and Persian calendars, you're asked to explain the calendar, whereas for the Gregorian calendar you are asked to explain the origins.

The answers to this are bookwork, so I'm not going to go into detail. The main point is that you are asked to explain them carefully, which means that the details are important. For example, for the Islamic calendar you need to explain

  • it is a pure lunar calendar (no synchronisation with the solar year);
  • months begin with the first visible crescent (not with the astronomical New Moon);
  • it is officially observational, not calculated, and therefore months will vary in length irregularly as a result of both the variation in the length of the synodic month and observational factors such as weather, altitude of the observing point, etc.
A similar level of attention to detail is required in the other two cases.
[2007 Q1.]

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