1a. Celest. Sphere
1b. Pole Star
2. The Ecliptic
2a. The Sundial
3. The Seasons
4. The Moon (1)
4a. The Moon (2)
4b. Moon Libration
Section #1 Stargazers and Skywatchers described the observed motion of the Sun across the sky, in different seasons of the year. This section tries to explain what is seen.
If the Earth's axis were perpendicular to the ecliptic, as in the drawings here, the Sun's position in the sky would be halfway between the celestial poles, and its daily path, seen from any point on Earth, would stay exactly the same, day after day.
Each point on Earth would be carried around the rotation axis AB once a day. On the equator (point C) the sun would always rise until it was overhead, then again descend to the horizon. At the poles (A and B) it would always graze the horizon and never get away from it. Except at the pole, every point would be in the shadow half the time, when on the right of the line AB, and would experience night; the other half it would be in the sunlight, experiencing day. Because the motion is symmetric with respect to the line AB, day and night anywhere on Earth are always equally long, 12 hours.
Actually, the axis of rotation is not perpendicular to the plane of the orbit. Instead, it points towards the celestial pole, near the direction of the pole star, and makes an angle of about 23. 5 degrees with the perpendicular to the ecliptic. That makes life a lot more interesting--we get longer days in summer, longer nights in winter.
Equinox and SolsticeAs the drawing on the right shows, the angle between the Earth's axis and the Earth-Sun line changes throughout the year. Twice a year, at the spring and fall equinox (around March 21 and September 22--the exact dates may vary a bit) the two directions are perpendicular, even though the axis is tilted.
At two other a year, the angle between the axis and that perpendicular direction is as big as it can get. These are at the summer and winter solstices, when it reaches 23.5 degrees. In the summer solstice (around June 21) the north pole is inclined towards the Sun, in the winter solstice (around December 21) it faces away from it.
Let us look at the summer solstice first, with the Sun on the left.
Summer and WinterThe boundary AB between sunlight and shadow--between day and night--is always perpendicular to the Earth-Sun line, as it was in the example shown at the beginning.
But because of the tilted axis, as each point on Earth is carried on its daily trip around the rotating Earth, the part of the trip spent in daylight (unshaded part of the drawing) and in the shadow (shaded) are usually not equal. North of the equator, day is longer than night, and when we get close enough to the north pole, there is no night at all. The Sun is then always above the horizon and it just makes a 360-degree circuit around it. That part of Earth enjoys summer.
A mirror-image situation exists south of the equator. Nights are longer than days, and the further one gets from the equator, the larger is the imbalance--until one gets so close to the pole that the sun never rises. That is the famous polar night, with 24 hours of darkness each day. In that half of the Earth, it is winter time.
Half a year later, the Earth is on the other side of the Sun, that is, the Sun's position in the above drawing should be on the right, and the shaded part of the Earth should now be on the left (light and dark portions in the drawing switch places). The Earth's axis however has not moved, it is still pointed to the same patch of sky, near the star Polaris. Now the south pole is bathed in constant sunshine and the north one is dark. Summer and winter have switched hemispheres.
A big difference between summer and winter is thus the length of the days: note that on the equator that length does not change, and hence Spring, Summer, Fall, and Winter do not exist there (depending on weather patterns, however, there may exist a "wet season" and a "dry season"). In addition (as the drawing makes clear), the Sun's rays hit the summer hemisphere more vertically than the winter one. That, too, helps heat the ground, as explained further in section #4, "The Angle of the Sun's Rays."
At equinox, the situation is as in the first drawing, and night and day are equal (that is where the word "equinox" comes from)
Some interesting facts
If June 21 is the day when we receive the most sunshine, why is it regarded as the beginning of summer and not its peak? And similarly, why is December 21, the day of least sunshine, the beginning of winter and not mid-winter day?
Blame the oceans, which heat up and cool down only slowly. By June 21 they are still cool from the winter time, and that delays the peak heat. Similarly, in December the water still holds warmth from the summer, and the coldest days are still ahead.
And what about our distance from the Sun? It, too, varies, because the Earth's orbit around the Sun isn't an exact circle. We are closest to the Sun--would you believe it? --when it's cold winter for most people, around January 3-5. This may have an interesting implication for the origin of ice ages, as will be explained in section (7).
Questions from Users: Where on Earth is the boundary between summer and winter?
*** What if the Earth's axis were tilted 90° to the ecliptic?
*** Does Earth rotate clockwise or counterclockwise?
*** Does atmospheric refraction give polar winters extra daylight time?
*** Does any location on Earth get the same number
of sunshine hours each year? .
*** Unequal Seasons
*** The Sun and Seasons
*** "Zenial Days" on Hawaii
*** Asian tradition on the start of winter
*** One year of continuous sunlight?
And for something completely different: When does Jewish Sabbath begin far north?
*** Earth tilt and climate (1)
*** Earth tilt and climate (2)
Next Stop: #4 The angle of the Sun's Rays
Timeline Glossary Back to the Master List
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 29 March 2014