Astronomy FAQ 004

The solstice is not defined by the length of day or night, but by the path of the Sun through the sky. To understand this FAQ you need to understand the concept of celestial coordinates. It sounds daunting, but is similar to the concept of latitude and longitude that we use to describe positions on the Earth's surface. Astronomers use a similar system to describe postions in the night sky. FAQ 3 explains this. If you are not familiar with the idea of celestial coordinates I suggest that you read FAQ 3 first.

By considering where the sun is against the background stars we effectively stop the rotation of the Earth (phew! I was beginning to get dizzy). The path traced out by the Sun against the background stars is known as the Ecliptic. If the Earth's axis through the poles, the spin axis, was perpendicular to the plane of the Earth's orbit, then the Sun would always be overhead at noon on the Equator, and the ecliptic would coincide with the celestial equator. However the Earth's axis is tilted at an angle of 23½° relative to the plane of the Earth's orbit which causes the Sun to shine more on the northern hemisphere during our summer and more on the southern hemisphere during our winter.

Correspondingly, the part of the ecliptic that relates to our summer is north of the celestial equator and the other part is south. Where the ecliptic crosses the celestial equator the sun will be overhead at noon for anywhere on the equator, and at this time the night and day are equal length. This is therefore known as an equinox (equal-night). There are two equinoxes, one in the Spring, also known as the vernal equinox when the sun moves from the south celestial hemisphere to the north, the other in the autumn when it returns from the north to the south again.

As the sun moves along the ecliptic from the spring equinox it gets further and further north in the sky. When it approaches the autumn equinox it will be getting further south in the sky each day. This is why, for us in the northern hemisphere, the Sun is high in the sky in summer and low in the sky in winter. Some where, between the two equinoxes there will be the point where the Sun reaches the most northern point, which will be half way between the two equinoxes. This point is called the solstice (sun-stands still) and the latitude (approx 23½°) where the Sun is overhead at noon at this time is called the Tropic of Cancer. The corresponding one in the south is called the Tropic of Capricorn. Just as there are two equinoxes, there are two solstices, the summer solstice where the Sun reaches its most northerly point, and the winter solstice where it reaches its southernmost point. I will consider the summer solstice but the winter one is similar excpet that it has a long night and a short day.

If you consider just the north south movement of the sun, then as it gets close to the solstice its movement northwards gets slower and slower, but it is always moving northwards until, at the solstice, just for a moment, it is stationary. It then immediately starts moving southwards. At that instant there will be one place on the Tropic of Cancer where it will be noon and the Sun will be overhead. This defines when the solstice is, that particular moment in universal time (UT). Note that it may not be noon at Grenwich, (though it could be by remote chance).

Since the Sun is above the horizon for longer in the summer than in the winter, as can be seen in the diagram where the light colours are when the sun is above the horizon and the dark colours when it is below, we get longer days in the summers and longer nights in the winters. The longest day will be the one in which the summer solstice occurs. If the solstice occurs during the day then that day is obviously the longest. If the solstice occurs during the night, then it will be the day before if the solstice is before midnight and the day after if the solstice occurs after mifnight. If the solstice occurs exactly on midnight then the days either side will be equal length and there will be two longest days that year.

The leap year does not have any effect on the solstice which is defined directly from the Earth's orbit around the Sun. However, the year is not an exact number of days, but 365¼ approximately. As we prefer to use a calendar with an exact number of days, the calendar date we assign to the solstice can vary by a day depending on whether we are nearly due for a leap day or not.

Provisos (for this FAQ)

1. This explanation, for simplicity, assumes that the Sun is at the centre of the Earth's circular orbit. In reality the Sun is at one focus of an eliptical orbit, and according to Keplar's laws of planetry motion the Earth does not travel at a constant speed around its orbit.
   
   
2. The point where the Sun crosses the celestial equator is not fixed but moves, very slowly around the ecliptic, the precession of the equinoxes. This is due to the wobble of the Earth on its axis. This has been ignored, but would be taken in to account by a decent planetarium program.