From the solstice onward, the length of day and night change. If you are in the Northern Hemisphere, yesterday was the summer solstice, which means that the day length is slowly but surely going to decrease over the next six months. But if you are a lover of sunsets, you might already know a particular puzzling fact: the latest sunset of the year happens days after the solstice rather than on the "longest day". What gives?
What gives is that what we use for our everyday measurements such as the length of day, are not exactly correct when taken day by day. Also, Earth’s orbit is close to but not exactly a perfect circle. And to top it all, Earth has a pretty spectacular tilt of 23.7 degrees, which is actually the reason why we have seasons. All of this combines to mess with our day-to-day length of day.
The usual 24 hours is what we call a day. Technically it is the mean solar time – the reason for the mean adjective will be clear in a second. A solar day is the time between the Sun passing over the same meridian. If you average all the days of one year, you would get 24 hours plus or minus some milliseconds. But the length of a singular day can change by many seconds, being longer at the solstices and shorter at the equinoxes.
As the real solar day lengthens around the solstice, our clocks experience it as a shift in the day length. So yes the number of daylight hours is slowly shrinking and sunrise is slightly later, but the real noon is slightly offset from the measured noon. So, for a few days, the sunset will happen later according to our clocks.
One of the issues here is that the Earth, as mentioned, is tilted, which creates the seasons with its equinoxes and solstices. On top of that, Earth’s orbit brings the planet periodically closer or further from the Sun. This variation in distance is about 5 million kilometers (3.1 million miles), with the furthest point (aphelion) happening in early July and the closest (perihelion) happening in early January. The Earth is moving at different speeds throughout the year.
Ultimately, the Sun and our averaged-out clock don’t agree. They don’t agree often, but it becomes more apparent when we are considering an extreme like the summer solstice.
If you like cool visualizations and Earth day curiosities, check out this cool video of how sunlight plays across the Earth in a year.