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Treebeard's Stumper Answer
24 March 2000

Equal Days?

The vernal equinox happened last week on March 20. It's spring! Days are now longer than nights north of the equator, and I'm thinking about summer! But consider the big picture. It's sunrise in the Arctic, it's autumn south of the equator, and it's twilight as the Sun sinks below the horizon for the 6 month winter night in Antarctica. Do all parts of the Earth receive the same number of daylight hours from the Sun over the year? Do all parts of the Earth receive the same warmth from the Sun? (Is that different?) How does this effect life?


All the Earth receives nearly the same sunlight over the year. Long summer days balance winter nights, even at the poles. But we are closest to the Sun in January, so we receive a bit more daylight during our summer north of the equator when the Earth is more distant and moving slower. Air bends sunlight by refraction, so we can still see the Sun even after it has set below the horizon. This is most noticeable at the poles where the low Sun stays close to the horizon. As a result, the Arctic has the most hours of daylight, but the higher sun near the equator gives the most heat.

Notes:

It's interesting that there was a full moon on the winter solstice (22 Dec 99) AND the vernal equinox (20 March 00)! The solar and lunar calendars are not in sync, so what gives?

If the Earth were a perfect sphere, and the Earth's orbit were a perfect circle around the Sun, and even if the Earth had no atmosphere, then the polar regions would still receive a bit more daylight than the equatorial regions. The Sun is so bright that we normally reckon the day from the first showing of the rising Sun to the last glimpse of the setting Sun. That's not symmetrical, so we get more day than night over the year. Even the equinox has more day than night. This is true everywhere on Earth, but near the poles, the Sun rises and sets at a more shallow angle and spends more time close to the horizon, so it has a greater effect. Twilight and sunrise/set is longer at the poles than the equator even without an atmosphere. This makes the polar days longer.

Refraction through the Earth's atmosphere exaggerates this effect even more. Sunlight is bent by the Earth's atmosphere just like a straw looks bent in a glass of water. The exact amount of refraction is about 34 minutes of arc on the horizon, just a bit more than the apparent diameter of the sun itself. The weird result is that when the sun appears to be sitting on the horizon, the real sun is entirely below it. We only see a virtual sunrise and sunset!

In fact, the Earth is not a perfect sphere, and its orbit is elliptical. The Earth is closest to the Sun (91.5 million miles at perihelion) on about January 4, and farthest from the Sun (94.5 million miles at aphelion) six months later on about July 4. Any planet's orbital speed is greater when it is closer to the sun, So our summer in the northern hemisphere is a bit longer than in the southern hemisphere. You can discover this fact on any calendar! On the other hand, the Earth is closer to the sun during the southern hemisphere summer, so the light is a bit more intense. Do these effects cancel out? Either way, I think the north pole gets the most daylight.

Because of refraction, the north and south poles receive more hours of daylight than the equator. And because of the Earth's elliptical orbit, the Arctic receives a bit more daylight than the Antarctic.

The total energy received from the Sun, called insolation, is a different matter. That depends on the elevation of the Sun in the sky as well as total hours. Shine a flashlight straight down. It makes a small bright circle. Now tilt the flashlight, and the circle of light spreads out into a larger ellipse. Because it is more spread out, the light (and heat) per unit of area is less intense.

On the equinox, the Sun follows the path of the celestial equator across the sky, but we see it higher or lower depending on our latitude. Point at the North Star with one arm, hold your other arm out at a 90 degree angle, and sweep it across the sky from horizon to horizon. That's the celestial equator. In summer and winter, the Sun will be 23.5 degrees higher and lower because of the Earth's tilt. At the north pole, you'll sweep around the horizon, and the sun will never be more than 23.5 degree high in the sky. The poles get the most daylight, but much less warmth because the Sun is so low.

The constant summer daylight is a boon for wildlife, but summer is short and intense. The polar regions get the most heat per 24 hour period of any place on Earth. (I think?) Migratory fish, whales, and birds know all about it. Winter is a different story, and most animals hibernate or leave. A few, like Emperor Penguins and a few humans, stick it out.

Insolation is a theoretical concept that clouds and climate change. I spent a week on the Kenektok River on the Bering Sea-side of Alaska a few years ago fishing with my Dad for the most beautiful Rainbow Trout I've ever seen. (It's strictly barbless flies, catch and release.) It rained every day. The Sun only came out for an hour that whole week, and the mosquitoes were so bad that I was happy when it started raining again! Climate dramatically effects insolation!

Water is a huge heat reservoir. It's significant that the continents are concentrated north of the equator, and there's more ocean to the south. It's significant that Antarctica is land, but the Arctic is ocean. How would the Earth be different if this was different?

The polar winter is harsh, but there's lots of daylight in the summer. Why doesn't the ice all melt in a perfect freeze/thaw cycle to match the day/night cycle? I think this has to do with the high albedo of ice. Ice reflects so much sunlight that it's harder to thaw than to freeze, and the poles have a net energy deficit despite more daylight. If by chance the Earth was to freeze over, it might stay that way!

I'm still confused by the equator. On the equinox at the equator, the Sun rises due east, climbs straight up to the zenith, and sets due west. I want to see that sometime! After the equinox, the Sun rises a bit to the north, and culminates north of straight-up. That's a longer path across the sky.

We performed an ancient ritual at Dunn Middle School on the vernal equinox. We drove an upright sun-stick into the ground and marked the shadow position every hour or so during the day. South is to the right, and the shortest shadow points north. The vernal and autumnal equinoxes are the only days of the year on which the shadow will track in a straight line from west to east as the Sun moves from east to west. I'm sure this is why so many ancient calendars begin on the equinox. Can you visualize how the Sun's shadow path will be different on the summer and winter solstices?

Here are some Web links for further research:

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