Longest day vs latest sunrise

01 June, 2009

Analemma
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.

Comments (5)

sort by
newest
oldest
Eric S 11 December, 2013 11:50
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. This information isn't correct. At solstices the solar day is LONGER, not shorter, than 24 hours. Otherwise, the sunsets would not be getting LATER after solstice. Everything else said is correct.
reply
jhon jopson 20 December, 2010 20:35
Are the good folk of Byron Bay expounding a myth when they say, 'The sun first rises there' or the rays of the sun strike Mt Warning before anywhere else in australia
reply
Discovery Centre 1 February, 2010 16:27

The time reported in the newspapers will be a time calculated for a particular point in Melbourne and will take into account its exact latitude and longitude. However this calculation does assume a perfectly flat horizon and standard atmospheric conditions. Most people do not look across a flat horizon and so will see the Moon (or Sun) rise a few minutes after the calculated time. Unusual atmospheric conditions on the horizon can also shift the apparent rising time a little.

reply
Ruth Slocum 31 January, 2010 20:38
Thankyou for answering my last question regarding the blue moon. My latest question is regarding Eastern Standard Time and the rising of the sun and moon. I gather EST refers to the whole of the eastern seaboard and states. But when looking on a map Melbourne is about halfway between Sydney and Adelaide. So if the papers say the moon is rising at 8.29pm (last night) that is applicable for those on the eastern seaboard. The moon, from where I watched it rise last night appeared approx 8.37ish. What is the actual delay for the rising of the celestial stars/moon/sun in Melbourne? Does this make sense?
reply
Heather 1 January, 2010 22:31
Thank you so much for this question and answer. I like to take note of when the solstices are, and had noticed that sunset gets later after the summer solstice, but had no way of figuring out why!
reply
Write your comment below All fields are required

We love receiving comments, but can’t always respond.

Image Gallery

Equation of Time