Seasons on Earth

Earth's Tilt

The fact that we have seasons (Spring Summer Fall Winter) on Earth is entirely due to the tilt of our North-South axis (by 23.5 degrees) relative to the orbit of the Earth around the Sun. This axis essentially remains fixed in space relative to the outside stars (thus it points very nearly to Polaris .. the North Star). Depending on where the Earth is in the orbit .. a given hemisphere will "see" the Sun as higher in the sky or lower in the sky. When the Sun is high in the sky, the weather is warmer ... summer! When the Sun is low in the sky, it is colder .. winter! (The reason for the seasons is often misunderstood to be the fact that the Earth is a little closer to the Sun part of the year, and farther away for the rest of the years. This is wrong - the closest the Earth gets to the Sun is actually in the Winter time .. and the farthest location is in the Summer time. Also, the difference in distance is only a few percentage points of the average distance .. this would not be enough to account for the large temperature changes between the seasons!)

Diagram of the Seasons (and the Perihelion and Aphelion)

Equinoxes and Solstices (oh my)!

During the Equinoxes, the Earth has equal day and nights. During the Summer Solstice, the day is as long as it can get (and the night as short). In the Winter Solstice, the day is short, and the night is long. These are also explained by the tilt of the axis ... during the Summer Solstice, the Northern Hemisphere faces the Sun as much as it can (bottom of the figure above) - as it moves around the orbit to the Fall Equinox, it is not facing the Sun as much. By the time of the Winter Solstice (since the NS pole still points the same way relative to the outside stars) the Northern Hemisphere now only sees the Sun in a more glancing way (it will be low in the sky).

A Hands-On model - showing Seasons and Equinoxes/Solstices and Lunar Phases

Ingredients

• Light bulb (or some object) to be the Sun
• Mini globe (or full size one) of the Earth
• Small ball for the Moon (KMart sells a small half black/half yellow ball that is perfect!)
• Paper signs for Polaris (put on ceiling if possible) and Seasons with Zodiac names (put around the room)

Setting up the model

• Put the Sun in the center of the space you will use
• Put Polaris sign high on the wall, or on the ceiling if possible - set it up so that the Earth at the Spring location would point its North pole to Polaris
• Designate 4 points around a circle around the Sun - these are the Season points - put the cards with the Season names and the Zodiac names
  • Summer = Aquarius, Capricornus, Sagittarius - arrow from right to left (i.e. Sagittarius .. to Capricornus .. to Aquarius)
  • Fall = Pisces, Aries, Taurus
  • Winter = Leo, Cancer, Gemini
  • Spring = Scorpius, Libra, Virgo

How to use the model

• Looking down from above, Earth revolves around the Sun in a counter-clockwise manner - moves through the seasons
• North pole of the Earth should always be pointed the same way (toward Polaris, ideally)
• As the Earth moves around the Sun - rotate the Earth on the axis in that same sense
  • Note how well the Northern Hemisphere can "see" the Sun ... very well in Summer, not so well in Winter
• As Earth moves around the Sun, notice what the "midnight side" of the Earth sees ... different constellations depending on where the Earth is in the orbit
• Put the Earth in the Summer position - Northern Hemisphere sees the Sun "high" in the sky
  • Bring the Moon in orbit around the Earth - start with it farthest from the Sun
    • Notice that as the Earth rotates on the axis - the Sun is high in the sky .. what about the Moon?
      • It looks "low" in the sky, just like the Sun appears in the winter time
• Put the Earth in the Winter position and do the same as above (note Moon is "high" when Sun is "low")
• Use your head at a position "fixed" in its orbit, move the Moon around your head in an orbit that has the same "sense" as the Earth's orbit (counter-clockwise from above)
  • Keep the yellow half of the moon always facing the Sun
  • Pick four locations around the orbit to "observe" the sun (the closest to the Sun, the farthest, and either side)
  • At the farthest from the Sun, we see the whole bright side of the Moon - Full Moon! (If Moon lines up with Earth-Sun = lunar eclipse)
  • At the closest to the Sun, we see the "dark side" of the Moon - New Moon! (If Moon lines up with Earth-Sun = Solar eclipse)
  • When "ahead" of the Full moon .. we see a crescent - with the bright side pointing to the Sun
• Using your head as the Earth, you can rotate (counter-clockwise from the top) and "observe" the sky changing - have someone hold the Moon for you
  • Depending on where the moon is in its phase .. does it show up near sunset, sunrise, midnight, noon?
    • New Moon - near noon, Full - near midnight, waxing (getting more full) - sunset, waning (getting more New) - sunrise

Another Model for how our rotating Earth sees the Sun and the Stars (and how we explain the Equinoxes and Solstices)

I will describe a hands-on demo that you can use to explain a wide variety of effects of the Earth's rotation. The easiest way to do it is with overhead transparency sheets. (Thanks to Michael Lopresto of HFCC for this model!)

How to use the model:

• Put the Earth layer down first .. then the Observer layer
• Rotate the Observer layer so that the Zenith points to the angle of the desired latitude (about 45 degrees would work for Detroit)
• Put the Sun layer parallel to the Equator - it is constrained to move only parallel to the Equator - and it must intesect the Ecliptic
• Start with the Sun line superimposed on the Equator .. this could be the Spring Equinox
  • Notice that the part of the Sun line that is "above" your horizon is as long as the part that is "below" your horizon - equal day/night!
• Move the Sun line up so that it touches the point where the Ecliptic touches the upper hemisphere - this is the Summer Solstice
  • Notice the Sun line "above" your horizon is much larger than the one below (long day - short night)
• Move the Sun line back down to superimpose the Equator - this is the Fall Equinox
• Move the Sun line down to the lower limit - this is the Winter Solstice - notice the short day and long night!
• Now take a look at the two star lines .. they are parallel to the Equator, since their motion appears based on our rotation
  • The star line that is more near the Equator spends some time above the horizon .. but sometimes is below the horizon (that star can "set")
  • Notice the star line up higher near the pole ... that star aways stays above the horizon .. we call that a circumpolar star (never "sets")
    • For the Northern Hemisphere ... the Big Dipper is always in the sky ... but stars that are down near the Ecliptic will set