June 20, 2008
The Strange Science of Summer
With 8 inches of hail falling in parts of Nebraska this week and
Arizona reaching triple digit temperatures last week, it may seem
rather arbitrary to call June 20 the first day of the summer this year,
aka the summer solstice. But scientists really do have a reason.
It's all about Earth's cockeyed leanings and some celestial configurations that even the ancients understood.
Our planet is tilted 23.5 degrees
on its spin axis. On June 20 this year (some years it's June 21), the
North Pole is pointing toward the sun as much as is possible.
Imagine Earth as an apple sitting on one side of a table, with the
stem being the North Pole. Tilt the apple 23.5 degrees so the stem
points toward a candle (the sun) at the center of the table. That's
summer for the top half of the apple. Now keep the stem pointing in the
same direction but move the apple to the other side of the table: Now
the stem points away from the candle, and it's winter on the top half
of the fruit.
The setup at June solstice puts the sun as high in our sky as it can
go, yielding the longest day of the year in the Northern Hemisphere.
Scientists put the exact moment of the solstice at 8:00 p.m. ET
(keep in mind that the sun is always up somewhere, and the gods don't
favor the Eastern time zone).
As long ago as the fourth century B.C., ancient peoples in the
Americas understood enough of this that they could create giant
calendars driven by sunlight. They built observatories of stone to mark the solstices and other times important for planting or harvesting crops. Shrines and even tombs were also designed with the sun in mind.
The sun comes up each day (except at the poles) because our planet
rotates once on its axis every 24 hours or so. It is Earth's tilt, and
our 365-day orbit around the sun, that explain much about how our world
changes during the year.
Seasons: As Earth orbits the sun, the orientation of the planet's axis, in relation to the sun, changes constantly.
A quarter of the way around in the orbit, fall sets in. By winter,
we'll be on the other side of the sun, with the North Pole pointing
away from the sun. That winter solstice, around Dec. 21 each year, will
be the Northern Hemisphere's shortest day, and researchers in
Antarctica will be basking in 24-hour sunlight.
Shifting stars: As we orbit the sun, the part of the night sky that's in our view changes.
A given star sets about 4 minutes earlier each night. Over a month,
this amounts to two hours. In winter, this all means that we're looking
at stars that during the summer were in our daytime sky, overwhelmed of
course by the glare of the sun. Since we complete a circle every year,
the stars of summer, such as the Big Dipper, are always the stars of
Endless summer: At the North Pole, the sun rises
once a year, around March 19. It rises until the summer solstice, then
sinks but does not truly set until around Sept. 24.
During summer on the top half of Earth, our planet is actually
farther from the sun than during winter, a fact owing to our
non-circular orbit around the sun. The difference is about 3 million
miles (5 million kilometers), and it makes a difference in radiant heat
received by the entire Earth of nearly 7 percent. But the difference is
more than made up for by the longer days in the Northern Hemisphere
summer with the sun higher in the sky.
Which brings up a common question: If the June solstice is the
longest day of the year, why are the dog days of August typically
hotter? It takes a while for the oceans to warm up, and a lot of weather on land is driven by the heat of the oceans.