Mark Merwood, of Lower Hutt, asks :-

At mid-winter in Wellington (latitude 41.3 degrees), the Sun rises at 7.48 am and sets at 4.58 pm (NZST), whereas in Sydney (latitude 33.9 degrees), the Sun rises at 7:00 am and sets at 4.54 pm (AEST). Sunrise in Sydney is earlier at that time of year, because it’s further north. However, the sunset times are almost the same. Why isn’t the Sydney sunset time later than that for Wellington?

Duncan Steel, a space researcher, author and broadcaster (duncansteel.com) who lives in Wellington, responded.

The latitude of a city affects the time at which sunrise and sunset occurs. The further you are from the equator, the greater the seasonal variation in the length of daytime as compared to the duration of darkness. However, the time of day on your clocks also depends on your longitude.

The time indicated on your wristwatch or computer screen is not directly linked to the position of the Sun in the sky because we use standardised time zones. NZST (New Zealand Standard Time) is defined to be twelve hours ahead of Coordinated Universal Time (UTC), which is a modernised version of what was known as Greenwich Mean Time (GMT). Those twelve hours correspond to 180 degrees of longitude east of the Greenwich Meridian (the north-south line passing through the old Royal Greenwich Observatory in London), whereas

Wellington is located at longitude 174.8 degrees, 5.2 degrees west of 180 degrees. In consequence the Sun does not transit Wellington’s meridian until almost 21 minutes after it passed the 180-degree longitude. One could say that the capital’s clocks, when set on NZST, are 21 minutes ahead of themselves; and when daylight saving time (NZDT) is in operation, the clocks are 81 minutes ahead of the time appropriate for Wellington’s longitude.

To the contrary Sydney uses, during the winter, AEST (Australian Eastern Standard Time) which is ten hours ahead of UTC and therefore is appropriate for a longitude of 150 degrees, whereas Sydney’s longitude is 151.2 degrees, equivalent to almost five minutes behind the Sun’s position.

Back to Wellington. Subtract 21 minutes from the 07:48 and 16:58 and you get 07:27 and 16:37. Mid-way between those times is noon, according to the Sun: 12:02. That’s the time the Sun transits the Wellington meridian at mid-winter (i.e. the solstice in June), according to the Sun itself. According to your wristwatch, though, the time would be 12:23 (i.e. that would be the time on NZST).

Similarly for Sydney. Add five minutes onto 07:00 and 16:54 to get 07:05 and 16:59, and mid-way between those is again 12:02, and the Sun does not lie! Your wristwatch working on AEST would display 11:57, however.

The above might prompt another question: why is the Sun crossing the meridian at two minutes after noon? The answer is that for timekeeping we use a theoretical concept known as the ‘Mean Sun’, which is an average position of the Sun in the sky. This is the origin of the word ‘Mean’ in Greenwich Mean Time.

The Mean Sun varies during the year by up to 16 minutes from the location of the actual Sun, that being equivalent to eight times its apparent diameter in the sky. It happens that at the June solstice the Mean Sun is only two minutes away from the real Sun.

This variation between the real Sun (and so the time indicated by a sundial) and the Mean Sun (the time scale we use for everyday timekeeping) is termed ‘The Equation of Time’. Two factors contribute to it. The first is the tilt of Earth’s spin axis, causing the Sun to move north and then south again during the year. The second is the non-circularity of Earth’s orbit, causing us to move slightly sunwards between about July 4th (when our planet is furthest from the Sun) until about January 4th (when we are nearest the Sun), and then to edge away again slowly over the next six months.