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. If you were to wait six hours and then look for the Sun

A. you find that it is still due southB. you would find that it has moved to the east

because of the Earth's rotationC. you would find that it has moved to the west

because of the Earth's rotationD. 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

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

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

• 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

Tilt of Earth’s axis

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

To do

• Lab next week: Seasons!• Canvas homework: science and pseudoscience