Astronomy by eye: motions in the Sky Reflex motion from Earth’s rotation Reflex motion from Earth’s revolution

Recap Science and pseudoscience/astrology Canvas assignment on science/pseudoscience due next Wednesday (to be posted later today) Astronomy by eye: motions in the sky By eye, we can’t tell how far away objects are, so we just talk about what direction they are in Celestial sphere Astronomical longitude (right ascension) and latitude (declination) Objects can appear to move because of intrinsic motion and/or reflex motion Reflex motion from Earth’s rotation: celestial sphere appears to spin around once per day Apparent motion in sky depends on declination of object

What’s the best description of Earth’s rotation and motion of stars? A. stars appear to move E to W because the Earth rotates from E to W B. stars appear to move E to W because the Earth rotates from W to E C. stars appear to move W to E because the Earth rotates from E to W D. stars appear move W to E because the Earth rotations from W to E E. stars don’t appear to move

Reflex motion from Earth’s rotation Depending on where you live on Earth, you might be able to see objects move around their full circles, none of the circles, or part of their circles You can only see stars when they are above the horizon Of course, you can’t easily see the stars when the Sun is up, because the light from the Sun overwhelms the light from the stars, but they are still there during the daytime

View from the North Pole Half the stars, all the time!

View from the equator All the stars, half the time!

View from Las Cruces Some stars all the time, some stars some of the time, some stars not at all!

An animation

Imagine you go out at noon, and you look due south and find the Sun Imagine you go out at noon, and you look due south and find the Sun. If you were to wait six hours and then look for the Sun A. you find that it is still due south B. you would find that it has moved to the east because of the Earth's rotation C. you would find that it has moved to the west because of the Earth's rotation D. you would find that it has moved to the west because the Earth has moved partway in its orbit around the Sun

Imagine you go out at midnight, and you look due south and find the constellation Scorpio. If you were to wait six hours and then look for Scorpio, A. you find that it is still due south B. you would find that it has moved to the east because of the Earth's rotation C. you would find that it has moved to the west because of the Earth's rotation D. you would find that the stars in Scorpio are no longer in the same place in the sky because they have moved in their orbits around the center of the Milky Way

If you could see stars during the day, this is what the sky would look like at noon on a given day. The Sun is near the stars of the constellation Gemini. Near which constellation would you expect the Sun to be located at sunset? A. Leo B. Cancer C. Gemini D. Taurus E. Pisces

A different perspective Imagine you lived in Santiago, Chile, which, at 33 degrees south latitude is about as far below the equator as Las Cruces is above it. From Santiago, the Sun would A. appear to move left to right across the sky, which is E to W B. appear to move right to left across the sky, which is E to W C. appear to move left to right across the sky, which is W to E D. appear to move right to left across the sky, which is W to E E. never rise or set

Telling time from the stars Since the stars revolve once around the sky for each rotation of the Earth, their motions can be used to tell time at night: they move one full circle around the sky each day

This is a real picture taken by putting a camera on a tripod pointed in the direction of the North celestial pole, and leaving the shutter open. How long did it take to make this picture? A. about an hour B. about 2 hours C. about 5 hours D. about 11 hours

Can you do it? Point to the celestial North Pole Point to the celestial equator Can you do this if you were at the Earth’s North pole? The Earth’s equator?

Reflex motion from Earth’s revolution In addition to rotating on its axis, the Earth revolves around the Sun, once a year As a result, the Sun appears to move with respect to the stars it passes through the constellations of the zodiac Of course, we don’t really see this because we can’t see the constellations when the Sun is up! Because of the Earth’s revolution, we see different stars at night over the course of the year

Tilt of Earth’s axis The Earth’s revolution around the Sun defines a plane The Earth’s rotation axis is tilted, by 23.5 degrees, relative to the plane in which the Earth revolves around the Sun The direction of the tilt is fixed in space, so it defines some special places in the Earth’s orbit: Solstices when the north pole is pointed towards or away from the Sun (although it never points directly at the Sun) Equinoxes when then the rotation axis is pointed perpendicular to the direction of the Sun

Path of the Sun in the Sky Because of this tilt, the latitude (declination) of the Sun changes over the course of a year Since the motion of an object across the sky depends on its declination, the motion of the Sun across the sky changes over the course of a year

Sun’s position over the course of a year at the same time each day: the Analemma

Seasons Because of the change of the Sun’s path across the sky over the course of a year, we have seasons: the temperature is different at different times of the year Why?

Imagine you are outside at the pool with the Sun overhead and you want to maximize the tan you get on your back. Which of the following positions would you choose? Why?

Imagine you are at at tanning salon and want to maximize the tan you get on your back. Which of the following positions would you choose to put the light? Note that all positions have the light at the same distance from your back.

Seasons: concentration of sunlight When the Sun is higher in the sky, its light is more concentrated on the Earth, so it is warmer Also explains why it is warmer at lower latitudes on Earth than at higher latitudes This concentration of sunlight has nothing to do with the distance of the Earth from the Sun

Seasons: length of day When the Sun is higher in the sky, it stays above the horizon for longer, contributing to making it warmer Concentration is more important, however: during summer at the North Pole, the Sun is up for 24 hours, but it’s still colder up there because the Sun is low in the sky the entire time! Useful animation? http://www.edumedia-share.com/media.php?id=964

Seasons: location on Earth Because of the tilt of the Earth’s rotation axis, the latitude (declination) of the Sun changes over the year When the Sun is at northern declinations, it is higher in the sky in the northern hemisphere, but lower in the sky in the southern hemisphere! When the Sun is at southern declinations, it is lower in the sky in the northern hemisphere, but higher in the sky in the southern hemisphere! As a result, seasons are reversed between the two hemispheres: summer in north is winter in south, and vice versa

Seasons: distance of Earth from Sun Seasons on Earth have NOTHING TO DO with changing distance of Earth from Sun The Earth’s orbit is almost a perfect circle, the small deviations from this have only a tiny effect on the temperature. In fact, the Earth is slightly closer to the Sun during northern winter! While the tilt of the Earth’s axis causes some portions of the Earth to be ever so slightly closer to the Sun, this has a negligible effect on the temperature. Remember, the size of the Earth is tiny compared to the distance from the Sun!

Seasons on other planets Seasons result from the tilt of the rotation axis relative to the plane of revolution around the Sun Different planets have different tilts, hence different seasons! Jupiter: tilt is nearly zero! Uranus: tilt is nearly 90 degrees Mars: tilt is about 23 degrees, just like Earth But Mars is more complicated, because its orbit is actually significantly non-circular, which is different from the Earth!

Reflex motion of stars from Earth’s revolution We talked about reflex motion of stars and Sun from Earth’s rotation (circles in the sky) We talked about reflex motion of Sun from Earth’s revolution (different constellations at different times of year, seasons) What about reflex motion of stars from Earth’s revolution? We’ve talked about this already too! Parallax!

The rising and setting of the Sun each day is caused by: A. intrinsic motion of the Sun around the Earth B. reflex motion from Earth rotating on its axis C. reflex motion from Earth revolving around the Sun D. intrinsic motion of the Sun around the center of the Milky Way

The circular motion of the stars around the North Star is caused by: A. intrinsic motion of the stars around the North Star B. reflex motion from Earth rotating on its axis C. reflex motion from Earth revolving around the Sun D. intrinsic motion of the stars around the center of the Milky Way

The changing of constellations that can be seen at different times of year is caused by: A. intrinsic motion of the stars around the Earth B. motion of Earth rotating on its axis C. motion of Earth revolving around the Sun D. intrinsic motion of the stars around the center of the Milky Way

Parallax occurs as a result of: A. intrinsic motion of the Sun around the center of the Milky Way B. reflex motion from Earth rotating on its axis C. reflex motion from Earth revolving around the Sun D. intrinsic motion of the stars around the center of the Milky Way

Seasons occur as a result of: A. the rotation of Earth on its axis B. the changing distance of the Sun from the Earth as the Earth revolves around the Sun C. the tilt of the Earth’s rotation axis relative to the plane in which it revolves around the Sun D. variation in the temperature of the Sun at different times of year

To do Lab next week: Seasons! Canvas homework: science and pseudoscience