Analemma image taken over the course of a year by Robert Price in Bethanga, Victoria, consisting of 48 images of the Sun superimposed on a single background image.
Image: Robert T. Price
Source: Robert T. Price
Question: Why is the day with the latest sunset not the same as the longest day?
Answer: The latest sunset and earliest sunrise do not happen on the longest day because sun time is not the same as clock time. The longest day has the longest time between solar noon and sunset, while the latest sunset has the longest time between clock noon and sunset.
Solar noon, also called local noon, is when the Sun reaches its highest point in the sky for the day. Clock noon happens at 12 o'clock every day, halfway through the 24 hour period. Clocks use 24 hours for a day because 24 hours is the average length of a solar day over the entire year. However individual days can be longer or shorter than 24 hours.
There are two factors which make the length of the solar day vary: the Earth's orbit is not perfectly circular, and the tilt of the Earth's axis relative to its orbit around the Sun. The combination of these gives the total amount by which solar noon is earlier or later than 12 noon and is known as the "Equation of Time".
At both the summer and the winter solstice, the solar day is shorter than 24 hours, and so the time of solar noon gets earlier each day. As a consequence, the day with the latest sunset – the longest time between clock noon and sunset – occurs about two weeks after the summer solstice, and the day with the earliest sunrise occurs about two weeks before it.