Sun-Moon hour angles

At the request of Mr. Heiner Thiessen, I have prepared data files which contain the difference between the geocentric hour angles of the Sun and Moon at two-hour intervals. These files should be used in conjunction with Mr. Thiessen's article in the June 2003 issue of the journal of the British Sundial Society.

The files cover the following date ranges:

Format of the files

The files are plain text. Here is a brief excerpt:

2003-06-01 00:00     9.57
2003-06-01 02:00    10.58
2003-06-01 04:00    11.60
2003-06-01 06:00    12.62
2003-06-01 08:00    13.64
2003-06-01 10:00    14.66

Each line contains a date (in ISO format: YYYY-MM-DD), a time and an angle. All times are expressed in Universal Time, which is equivalent to Greenwich Mean Time. This is the standard time system used by professional astronomers.

The meaning of the angles

The angle is the difference of hour angles in the sense Sun-minus-Moon. It is positive when the Moon is waxing (east of the Sun, transiting after the Sun), and negative when the Moon is waning (west of the Sun, transiting before the Sun).

There is an apparent discontinuity around Full Moon, as this excerpt for 14 June 2003 illustrates. Full Moon occurs at 11:16 Universal Time on this date. Note how the sign of the angle is positive before the instant of Full Moon and negative afterwards. This is simply a consequence of the convention which restricts the range of values from -180° to +180°.

2003-06-14 06:00   176.54
2003-06-14 08:00   177.81
2003-06-14 10:00   179.07
2003-06-14 12:00  -179.66
2003-06-14 14:00  -178.38
2003-06-14 16:00  -177.11

For the purpose of interpolation, the angle at 12:00 can be added to 360° to yield a positive value, +180.34°.