Okay I know this is like 4th grade science or something, but I think with my stroke I just can’t seem to picture why the sun is “on” or “off” for most of the day up at the North Pole or down at South Pole, and yet at the equator days are very regular year ’round.
I keep picturing the Earth on its axis, but I just can’t seem to figure out how the sun shines differently at different latitudes.
In: 3
There’s two ways to look at this – from space and from the ground.
From space, the Earth’s axis is tilted relative to its orbit. So in Northern summer, the north pole is tilted towards the Sun. The north pole itself doesn’t move as the Earth rotates, and the land near it doesn’t move by enough to ever end up on the night side of the planet (that is, the side facing away from the Sun).
But the more interesting picture, I think, is from the ground.
You probably think of the Sun as rising in the east, rising up directly overhead, and setting in the west. But that picture turns out to only be true at the equator on the equinoxes. Everywhere else on Earth, the story’s a bit different.
Instead, the Sun follows a circular track in your sky each day. (Technically, it’s not quite a circle, but it’s close enough for our purposes here.) At the equator, that track is oriented straight up and down – that is, if you imagine filling in that circle to make a disc, the disc would be a vertical flat plane. As you move away from the equator, though, that disc tilts by an angle equal to your latitude. At a typical latitude of most people reading these words – around 30 or 35 degrees North – the circle is tilted by 30 or 35 degrees. At the equinox, the Sun still rises in the east, but it follows a *slanted* path, peaks about 30 or 35 degrees *below* the point directly overhead (the *zenith*), and comes back down along a slanted path to set in the West.
If the Earth had no tilt to its spin, that would be the whole story. But because it does, we also have seasons. The seasons cause the Sun’s track to shift in the sky towards the north or south celestial pole – that is, the point in the sky you’d find if you took the north pole or south pole of the Earth and projected it out into infinity.
During northern hemisphere summer, the circle the sun follows “contracts towards the north celestial pole”. Since this pole is above the horizon in the northern hemisphere, this causes the Sun to be up for more than half of the day (since its track is “pulled” towards a point above the horizon). The Sun therefore rises somewhat north of due east, rises up into the sky, peaks in the southern sky, then follows a slanted path down to set north of due west. It dips below the northwest horizon, passes under the northern horizon overnight, and rises again in the northeast horizon the next morning.
If you’re far enough north, though, the Sun’s track is VERY tilted. If you’re at, say, 80 degrees north, the Sun’s track is only tilted by 10 degrees. That means that at the equinox, the Sun was only ever 10 degrees above or below the horizon. But the maximum effect of the seasons (23.5 degrees, which is the tilt of the Earth’s axis) is larger than that. So the Sun ends up being pulled from 10 degrees below the northern horizon at the equinox to 13.5 degrees **above** the northern horizon at the summer solstice. The entire track sits above the horizon, and you get the midnight sun.
During northern hemisphere winter, the opposite happens. The sun’s track moves *away* from the northern celestial pole in the sky. At temperate latitudes, the Sun now rises *south* of due east, rises to peak in the southern sky, and sets *south* of due west. It passes under the western, northwestern, northern, northeastern, and eastern horizons before rising again in the northeast the next day, and is up for much less than half the day.
If you’re far enough north, this effect can pull the entire sun’s track *below* the horizon, resulting in a polar night.
In short, there are two main factors that contribute to the Sun’s highest and lowest point each day:
* Your latitude. The Sun’s “baseline” highest point is (90 minus your latitude) degrees above the horizon, and its “baseline” lowest point is (90 minus your latitude degrees) below it.
* The seasons. During your hemisphere’s summer, the highest and lowest points rise by as much as 23.5 degrees. At the equinox, it follows its baselines. And during your hemisphere’s winter, the highest and lowest points *fall* by as much as 23.5 degrees. This pattern roughly follows a sine wave over the course of a year.
(There are some other minor factors, too, but this is enough to explain most of the seasons.)
Latest Answers