Why do we have the seasons?
As the earth spins on its axis, producing night and day, it also moves about the sun in an elliptical (elongated
circle) orbit that requires about 365 1/4 days to complete. The earth's spin axis is tilted with respect to its orbital
plane. This is what causes the seasons. When the Earth's axis points towards the Sun, it is summer for that hemisphere.
When the Earth's axis points away, winter can be expected. Since the tilt of the axis is 23 1/2 degrees, the north
pole never points directly at the Sun, but on the summer solstice it points as close as it can, and on the winter
solstice as far as it can. Midway between these two times, in spring and autumn, the spin axis of the Earth points
90 degrees away from the Sun. This means that on this date, day and night have about the same length: 12 hours each,
more or less.
Why should this tilt of the Earth's axis matter to our climate? To understand this, take a piece of paper and a
flashlight. Shine the light from the flashlight straight onto the paper, so you see an illuminated circle. All the light
from the flashlight is in that circle. Now slowly tilt the paper, so the circle elongates into an ellipse. All the
light is still in that ellipse, but the ellipse is spread out over more paper. The density of light drops. In other
words, the amount of light per square centimeter drops (the number of square centimeters increases, while the total
amount of light stays the same).
The same is true on the Earth. When the Sun is overhead, the light is falling straight on you, and so more light
(and more heat) hit each square centimeter of the ground. When the Sun is lower in the sky, the light gets more
spread out over the surface of the Earth, and less heat (per square centimeter) can be absorbed. Since the Earth's
axis is tilted, the Sun is higher when you are on the part of the Earth where the axis points towards the Sun, and
lower on the part of the Earth where the axis points away from the Sun.
For the Northern Hemisphere, the axis points most toward the Sun in June (specifically around June 21), and away
from the Sun around December 21. This corresponds to the Winter and Summer Solstice (solstice is Latin for "the sun
stands"), or the midpoints of winter and summer. For the Southern Hemisphere, this is reversed.
For both hemispheres, the Earth is 90 degrees away from the sun around March 21 and then again around September 21.
This corresponds to the Fall and Spring Equinox (equinox is Latin for "equal night"). Everyplace in the world has about
12 hours of daylight and 12 hours of night.
So why are sunrise and sunset not exactly 12 hours apart on the Equinox?
Day and night are not exactly of equal length at the time of the March and September equinoxes. The dates
on which day and night are each 12 hours occur a few days before and after the equinoxes. The specific dates for
this occurrence are different for different latitudes.
On the day of the equinox, the geometric center of the Sun's disk crosses the equator, and this point is above
the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise
is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset
is the instant when the trailing edge of the disk disappears below the horizon. At these times, the center of the disk
is already below the horizon. Furthermore, atmospheric refraction (or bending) of the Sun's rays cause the Sun's disk to
appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning, the upper edge of the
disk is visible for several minutes before the geometric edge of the disk reachs the horizon. Similarly, in the evening,
the upper edge of the disk disappears several minutes after the geometric disk has passed below the horizon.
For observers within a couple of degrees of the equator, the period from sunrise to sunset is always several minutes
longer than the night. At higher latitudes in the Northern Hemisphere, the date of equal day and night occurs before the
March equinox. Daytime continues to be longer than nighttime until after the September equinox. In the Southern
Hemisphere, the dates of equal day and night occur before the September equinox and after the March equinox.
When are the times and dates of the next equinoxes and solstices?
The chart shown below shows the dates and times for the equinoxes and solstices through 2012. Times listed are
in Mountain Standard Time.
|Year||Spring Equinox||Summer Solstice
||Fall Equinox||Winter Solstice
|2008 ||Mar 19 -- 10:48 pm ||June 20 -- 4:59 pm ||Sept 22 -- 8:44 am ||Dec 21 -- 5:04 am
|2009 ||Mar 20 -- 4:44 am ||June 20 -- 10:45 pm ||Sept 22 -- 2:18 pm ||Dec 21 -- 10:47 am
|2010 ||Mar 20 -- 10:32 am ||June 21 -- 4:28 am ||Sept 22 -- 8:09 pm ||Dec 21 -- 4:38 Pm
|2011 ||Mar 20 -- 4:21 pm ||June 21 -- 10:16 am ||Sept 23 -- 2:04 am ||Dec 21 -- 10:30 pm
|2012 ||Mar 19 -- 10:14 pm ||June 20 -- 4:09 pm ||Sept 22 -- 7:49 am ||Dec 21 -- 4:11 am
Is it true that you can stand an egg on end during the Spring Equinox?
The answer is YES. However, you can stand an egg on end, with a large amount of patience,
on any day of the year. This myth seems to pop up every year around the equinox, with the regularity
of the grass greening up in the spring. The thought that an egg can only stand on end on the spring equinox due to gravitational
forces of the sun being aligned with the earth sound like science, but it isn't. According to Chinese tradition, an egg
can be made to stand on end at the precise moment winter ends and spring begins. But, the Chinese calender had this
transition occurring at a variable time each year, determined partly by the Chinese lunar month, usually in early February.
More recently, this myth became fixed to the time of the spring equinox.
The underlying assumption relating to standing eggs on end is that there must exist some special gravitational balance.
There are many forces acting on an egg when you try to stand it on end on a flat surface. Some people think that the
gravitational pull of the Sun becomes balanced with those of the Earth to allow for this phenomenon to occur. However, the
moon exerts a much stronger gravitational effect on the Earth than the Sun, dominating the
ebb and flow of the ocean tides of the world. The moon's effects are different at each of the equinoxes however.
The most dominant force of gravity on a standing egg is the one between the Earth and the egg itself. This is determined
by the weight of the egg and the force pulling the egg to the counter top. To assume that some celestial balance occurs only
on the spring equinox (but not on the fall equinox!), is to fall prey to bad science.
If you want to prove this to yourself, take a fresh, uncooked, egg and hold it with the larger end resting on a
table or counter top. Wait for the fluid content of the egg to settle, then carefully test the balance. Be patient as
you find the point where you can ever so gently let it go to remain standing on end. If you want the experiment to be complete,
do it at various times of the year. Finally, next time you hear claims that sound miraculous, look for ways of
testing before accepting these claims blindly.