Sunset viewed from Sydney Observatory on 29 August 2005. Image and copyright Nick Lomb ©, all rights reserved
In 2011 spring equinox for the southern hemisphere is on Friday 23 September at 7:05 pm AEST. On that day the Sun crosses from the northern part of the sky to the southern and it is one of only two days in the year when the Sun rises due east and sets due west; the other being the autumn equinox in March. As on this occasion the exact time of the equinox is close to the time of sunset, the setting Sun will be almost exactly in alignment with azimuth 90° or west.
In many countries the change in the seasons is assumed to take place at the equinoxes and at the solstices in June and December. This is not the case in Australia where, for slightly obscure historical reasons, the seasons change at the beginning of the month so that spring has already began on 1 September. For whatever reasons we have that tradition, it does work well as the hottest days of the year do tend to occur in the middle of the December, January and February summer period. Similarly the coldest days tend to occur in the middle of the winter period of June, July and August.
The position of the Sun on the horizon when its centre is 90° from the zenith. Drawing and copyright Nick Lomb ©, all rights reserved
The most common question asked at the time of the equinoxes is why are there more than 12 hours of daylight on the day of the equinox? It is a good question for with the Sun rising due east and setting due west on the day, it would be sensible to assume that the length of daylight is halfway between its shortest in winter and its longest in summer.
In reality, there is always a little more than 12 hours of daylight on the day of the equinox. There are two factors involved. One is that all considerations involve the centre of the Sun. As indicated on the diagram above, when the geometric centre of the Sun is on the horizon, half of the disc is still visible. Sunset does not occur until that top half disappears below the horizon. As the angular size of the Sun is about 30′ of arc, an extra 15′ of arc has to be added for the calculation of sunset times.
The change in the apparent position of the Sun on the horizon due to refraction. Drawing and copyright Nick Lomb ©, all rights reserved
The second factor involved is the bending of light by the atmosphere or refraction. This means that near the time of sunset we see the Sun as above the horizon even though it is actually below. How far the sunlight is bent depends on both the temperature and the atmospheric pressure. An average value of 35′ of arc is normally assumed to be the extra motion the Sun has to undergo before it appears to set. This is, however, just an estimate and hence calculations of sunrise and set times are predictions only.
The two factors together add up to 50′ or arc so that the Sun has to be 90° 50′ from the zenith before it appears to set. This is the value assumed in calculations of sunrise or set times. To estimate how that extra 50′ of motion at both sunrise and set lengthens daylight note that to a rough approximation the Sun takes 4 minutes of time to move 1°. This suggests that at the equinox daylight will be 7 minutes longer than 12 hours. From the 2011 Australian Sky Guide the length of daylight on Friday 23 September can be worked out as 12 hours 8 minutes – close to our rough calculation.
If on the day of the equinox someone says that the times in the paper or in the sky guide are wrong, you can explain that they are exactly as expected after allowing tor the finite size of the Sun and for refraction.