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