Confusion over shortest day of the year explained

20 June 2011

Sunset near Urana New South Wales

Sunset near Urana New South Wales
© Geoscience Australia

The apparently odd behaviour of the Sun's rise and set times around the winter solstice is continuing to create confusion among many wanting to know why they don't coincide.

It is generally accepted that the winter solstice marks long cold nights and short periods of sunshine - but just when the phenomenon occurs generates considerable confusion.

The solstice is an astronomical event which occurs twice each year when the Sun's apparent position in the sky reaches its northernmost point in winter and southernmost extreme in summer.

The winter solstice usually occurs on 21 June but can vary to happen on the 20 or 22 June, although the date doesn't always coincide with the day having the least hours of daylight. This year, the solstice in eastern Australia is on 22 June at 3:16 am.

On the day of the winter solstice, to many people it seems odd that the time of sunrise continues to get later in the day after the solstice.

This discrepancy is because sunrise and sunset tables are calculated for the rising and setting of the Sun's upper edge rather than its centre and because the refraction, or bending, of the sunlight by the Earth's atmosphere is taken into account. The result of these two factors is that at the calculated instant of rising or setting, the Sun's centre is below the horizontal. If the rising and setting were computed for the Sun's centre exactly in the horizontal plane, neglecting refraction, the shortest day would coincide with the winter solstice.

Because the path of the Earth around the Sun is an ellipse, not a circle, and because the Earth is off-centre on its axis, these combined phenomena can create up to several minutes difference between solar and mean time.

Corrections for the difference between clock-defined noon and the time when the Sun is directly overhead or on the meridian, which is called the Equation of Time, represents the correction applied to the time given by a sundial to make it agree with clock time. This Equation of Time can change by up to 10 minutes from 16 June to 5 July, which means that the time when the Sun crosses the meridian changes by 10 minutes, resulting in the times of sunrise and sunset changing by the same amount.

A similar situation occurs at the time of the Equinox when the Earth is directly aligned to the Sun over the equator and at the time of the Aphelion in early July when the Sun is furthest from the Earth in the planet's elliptic orbit and at the Perihelion when the Earth is at its closest point to the Sun in early January.

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