If you live between the Tropic of Cancer and the Tropic of Capricorn, the amount of daylight you get each day doesn’t vary much. If you live near the equator, it’s pretty much thirteen-hour days and eleven-hour nights year round, with sunrise around 7:30 am and sunset around 8:30 pm. (You get a bonus hour of daylight because the sun’s rays are refracted above the horizon for a short time before the sun rises and after the sun sets.) However, if you live farther north or south, you’ll find that your days are a lot longer in summer and a lot shorter in winter. That’s because latitude affects the time of sunset and sunrise.
What’s going on?
Earth is tilted on its axis, by about 23 degrees or so. That’s why the parts of the Earth that are nearest the sun aren’t always the same.
The tilt always stays the same direction, so the North Pole always points towards Polaris, the North Star. That’s why Polaris is always the North Star. (Well, except for precession, but that takes thousands of years and isn’t important right now.)
At the same time, two other things are happening.
Earth’s spinning on its axis. That’s what causes day and night. Each part of the Earth experiences day when it’s pointed towards the Sun, and night when it’s pointed away from the Sun. It takes about twenty-four hours to make one complete revolution.
Sunrise happens when a place on the Earth reaches the sunlight as the Earth revolves. Sunset happens when a place on the Earth leaves the sunlight and enters the shadow cast by the Earth itself.
Earth’s also going around the Sun in its orbit. It takes about one year to make a complete orbit. The orbit means that the Earth’s axis isn’t always going to be pointed the same way relative to the Sun. Sometimes the daylight side of the northern hemisphere is closer to the Sun, and sometimes the daylight side of the southern hemisphere is closer to the Sun.
The combination of all these things means that at every time of year, some parts of the Earth get earlier sunrises and later sunsets than other parts. How much more daylight they see depends on their latitude.
June in the northern hemisphere
In June, the North Pole is at its closest to the Sun, and it’s pointing inwards, towards the Sun at an angle. When that happens, the whole northern hemisphere reaches the sunlight earlier and leaves it later as the Earth spins in space. When you’re looking at it from the surface of the Earth, the Sun rises earlier, and sets later.
That’s why every part of the northern hemisphere gets longer days and shorter nights, all summer long. The angle of the Earth’s axis means that sunrise is earlier as you go farther north, and sunset’s later. The longest day of the year is called the summer solstice. It’s usually around June 21.
It’s pretty extreme too. By the time you’re north of 45, you’ll get sunrises that are as early as 5:30 am, and sunsets around 9:30 pm or so. Once you get north of 60, the daylight’s nearly round the clock.
When you go farther north than that, things start to get really strange. By the time you reach the Arctic Circle, the days are around the clock, but it’s usually just for the summer solstice. If you go farther north, you’ll get more and more days that are round the clock sunlight in summer. At the North Pole, you’ll get six solid months of daylight!
June in the southern hemisphere
In the southern hemisphere, the opposite’s happening. The South Pole’s at its farthest from the Sun, so it’s winter down there. It gets stuck with six months of darkness, three months on each side of the winter solstice. Sunrise won’t happen until September.
That’s why the whole southern hemisphere reaches the sunlight later and leaves it earlier as the Earth spins in space. The angle of the Earth’s axis means that sunrise is later in the southern hemisphere in June, and sunset’s earlier. The shortest day of the year is called the winter solstice.
Polar nights are long, but they get shorter as you go farther north. You don’t get a ray of daylight on the winter solstice until you get north of the Antarctic Circle.
When you’re in the southern hemisphere in June, the Sun rises earlier as you go farther north. The Sun also sets later as you go farther north, right up until you reach the equator.
As you get closer to the equator, the winter days get longer and longer. Actually, it feels less and less like winter. By the time you reach the equator, the days are just about the same length as the nights. It varies a bit with the seasons, but not so you’d notice unless you’re paying really close attention.
In December, the South Pole’s at its closest to the Sun, so the southern hemisphere’s experiencing summer. At the same time, the North Pole’s farthest away, so it’s experiencing winter. It’s basically the exact opposite of what was happening in June.
The turnaround times between the northern and southern hemisphere are called the equinoxes, because that’s the time of year when the duration of days and nights are the closest to being the same. During these times, the North Pole and South Pole are about the same distance away from the Sun.
One equinox is around March 21. In the northern hemisphere, it’s called the spring equinox. The other equinox falls is around September 21. In the northern hemisphere, that’s the fall equinox.