Presentation: motions and cycles

Last week we used a starwheel in order to look at the positions of stars and constellations on the celestial sphere on a given date and time. While it may seem that the stars are already against you tonight, there is a lot more going on in the sky besides the positions of stars and constellations.

We'll take a look at four types of motions in the sky, with an emphasis here on distinguishing between them, and their effect on the positions of the stars and the sun. (We'll cover the motions and cycles of the moon in our next presentation.)

First, rotation.

Rotation is the spinning of Earth on its axis, and this cycle takes approximately 24 hours to complete (depending on your frame of reference.)

Rotation causes the stars and constellations (and even the sun!) to appear to move counterclockwise) around the celestial north pole, as seen in this Starry Night^TM simulation. (For the purposes of this presentation, we'll assume that we are always observing from San Luis Obispo, CA in the northern hemisphere.)

Second, precession.

Note that rotation is occurring every 24 hours as Earth spins on its axis. However, the direction of the axis wobbles like a top, and takes approximately 26,000 years to complete one cycle. Right now, the north end of Earth's axis points towards the star Polaris, but because of precession, the north end of Earth's axis will point towards other parts of the celestial sphere.

In this Starry Night^TM simulation, in 2700 B.C. the celestial north pole pointed towards the star Thuban, such that all stars and constellations moved counterclockwise around this "north star."

Because of precession causing Earth's rotation axis to wobble, currently the star Polaris gets to be the "north star."

Eventually precession will cause the star Er Rai to be the "north star" in 4400 A.D. And yes, there will more typically be no given "north star" when the north end of Earth's axis points towards an empty portion of the night sky.

Third, revolution. Previously you were asked to ponder the origin of the word "zodiac," or at least consider what other words it might be related to.

Like "zoo," or "zoology," the "zodiac" has something to do with animals, and is literally the "arc (or line) of animals." Traditionally there are twelve of them, but not all of them are now animals today.

Revolution is the motion of Earth around the sun, and this cycle takes one year to complete. Note that the zodiac constellations are approximately equally spaced, and if you could see stars and constellations during the day, the sun would directly line up on a different zodiac constellation on certain times of the year, here Virgo.

Approximately one month later, as Earth revolves around the sun, you would then see the sun directly lining up with Libra. One month after that, then Earth revolves further around the sun such that you would see the sun directly lining up with Scorpius.

This is the basis of "sun-sign" astrology, where it matters which zodiac constellation the sun lines up with at different times of the year. Again, this assumes you can see stars and constellations during the day, but this is not a big deal because you have your starwheels to figure out where the sun is on the zodiac right now. (In fact, let's do this now--and see if we notice anything...) In this time-lapse, the sun is seen at noon every day over a month, over which time the zodiac constellations that align with the sun shift over once (from Virgo to Libra).

Fourth, tilt. Which carmakers produce(d) the Solstice and the Equinox? Did any of you list these car models in your "Astronomy in the Marketplace" activity last week?

Pontiac made the Solstice sports coupe, and Chevrolet current makes the Equinox sports utility vehicle. (Did any of you list these astronomy-related cars in the "Marketplace Astronomy" in-class activity last week?) These are special times of year in terms of the position of the sun and the number of daylight hours, as we'll see in the in-class activity today. (The two equinoxes occur twice a year, when there are 12 hours of daylight (and 12 hours of night); the two solstices also occur twice a year, on the longest day of the year, and on the shortest day of the year.)

Note the tilt of Earth's rotation axis, with its north end (always) pointed towards Polaris. When Earth is located on the right side of its orbit around the sun, the northern hemisphere is tilted towards the sun, such that San Luis Obispo, CA in the northern hemisphere will see the sun high in in the sky, and it will be our summer.

So not just tilt, but revolution is needed to cause the seasons to change over the course of a year. Six months later, Earth will be over on the left side of its orbit around the sun, but as the north end of Earth's rotation axis is still pointed towards Polaris, this means that the northern hemisphere is tilted away from the sun, such that San Luis Obispo, CA in the northern hemisphere will see the sun low in in the sky, and it will be our winter.

Here is a very long time exposure of the sun making different paths across the sky over the course of the year, which makes a high arc in the summer, when our hemisphere is angled towards the sun, and makes a low arc in the winter, when our hemisphere is angled away from the sun. We'll cover this interesting set of paths across the sky in more detail in the subsequent in-class activity today.