1. Class Information for PHYS/ASTR 1050 can be found at:
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Everything about the class HW, Exams, Syllabus, etc can be found on the class Homepage.

2. The Solar System
Knowing the Heavens

4. The Night Sky 1/27/17 1-hr after sunset, Colorado Springs

5. Ursa Major — The Big Dipper
Figure 2-6 The Big Dipper as a Guide
The North Star can be seen from anywhere
in the northern hemisphere on any night of the year. This star chart
shows how the Big Dipper can be used to point out the North Star as well
as the brightest stars in two other constellations. The chart shows the
sky at around 11 P.M. (daylight savings time) on August 1. Due to the
Earth’s orbital motion around the Sun, you will see this same view at
1 A.M. on July 1 and at 9 P.M. on September 1. The angular distance from
Polaris to Spica is 102°.

6. The Winter Triangle Figure 2-7 The “Winter Triangle”
This star chart shows the view toward the
southwest on a winter evening in the northern hemisphere (around
midnight on January 1, 10 P.M. on February 1, or 8 P.M. on March 1). Three
of the brightest stars in the sky make up the “winter triangle,” which is
about 26° on a side. In addition to the constellations involved in the
triangle, the chart shows the prominent constellations Gemini (the Twins),
Auriga (the Charioteer), and Taurus (the Bull).

7. Why are stars tracing out circles in the sky?
Is the Earth rotating CCW about an axis?
or is the sky rotating CW around Polaris?
The Earth’s rotation makes stars appear to trace out circles in the sky.
(Gemini Observatory)

8. Question
Which of the following heavenly objects remains fixed in the sky relative to an observer's horizon?
Polaris, the north star.
the Moon.
the Sun.
Mars.
the Primum Mobile.

9. The Celestial Sphere — A crystalline sphere in which all the stars are embedded surrounds the Earth and rotates CW once every ~24 hr!

10. Latitude & Longitude Salt Lake City: 410 N. 1120 W.
Earth’s surface is 2-d
Require only 2 numbers to specify a location.
Those 2 numbers are angles.

11. Celestial Coordinates
Box 2-1 Celestial Coordinates
Your latitude and longitude describe where on the Earth’s
surface you are located. The latitude of your location denotes
how far north or south of the equator you are, and the
longitude of your location denotes how far west or east you
are of an imaginary circle that runs from the north pole to the
south pole through the Royal Observatory in Greenwich, England.
In an analogous way, astronomers use coordinates called
declination and right ascension to describe the position of a
planet, star, or galaxy on the celestial sphere.
Declination is analogous to latitude. As the illustration
shows, the declination of an object is its angular distance north
or south of the celestial equator, measured along a circle passing
through both celestial poles. Like latitude, it is measured
in degrees, arcminutes, and arcseconds (see Section 1-5).
Right ascension is analogous to longitude. It is measured
from a line that runs between the north and south celestial
poles and passes through a point on the celestial equator called
the vernal equinox (shown as a blue dot in the illustration).
This point is one of two locations where the Sun crosses the
celestial equator during its apparent annual motion, as we discuss
in Section 2-5. In the Earth’s northern hemisphere, spring
officially begins when the Sun reaches the vernal equinox in
late March. The right ascension of an object is the angular distance
from the vernal equinox eastward along the celestial
equator to the circle used in measuring its declination (see illustration).
Astronomers measure right ascension in time units
(hours, minutes, and seconds), corresponding to the time required
for the celestial sphere to rotate through this angle.

12. Stars You Can See Depend on Latitude
Figure The View from 35° North Latitude
To an observer at 35° north latitude
(roughly the latitude of Los Angeles, Atlanta, Tel Aviv, and Tokyo), the
north celestial pole is always 35° above the horizon. Stars within 35° of
the north celestial pole are circumpolar; they trace out circles around the
north celestial pole during the course of the night, and are always above
the horizon on any night of the year. Stars within 35° of the south
celestial pole are always below the horizon and can never be seen from
this latitude. Stars that lie between these two extremes rise in the east
and set in the west.

13. At North Pole, All Stars Above Horizon Never Set
Figure 2-11 The Apparent Motion of Stars at Different Latitudes
(b) At the north pole (latitude 90° north) the stars appear to move parallel to the horizon.

14. Stars Rise & Set at Middle Latitudes
Figure 2-11 The Apparent Motion of Stars at Different Latitudes
As the Earth rotates, stars appear to rotate around us along
paths that are parallel to the celestial equator. (a) As shown in this long
time exposure, at most locations on Earth the rising and setting motions
are at an angle to the horizon that depends on the latitude.
(David Miller/DMI)

15. On Equator, Stars Rise & Set ┴ to Horizon
Figure 2-11 The Apparent Motion of Stars at Different Latitudes
(c) At the equator (latitude 0°) the stars rise and set along vertical paths.

16. Day & Night Why? Is the Earth rotating CCW? or is the Sun rotating CW?
Figure 2-3 Day and Night on the Earth
At any moment, half of the Earth is
illuminated by the Sun. As the Earth rotates from west to east, your
location moves from the dark (night) hemisphere into the illuminated
(day) hemisphere and back again. This image was recorded in 1992 by
the Galileo spacecraft as it was en route to Jupiter. (JPL/NASA)

17. The Night Sky changes slowly throughout the year.
Because the Earth moves around the Sun once/year …
or the Sun moves slowly ‘backwards’ on the Celestial sphere around the Earth
Figure 2-5
Why the Night Sky Changes During the Year
As the Earth orbits around the Sun, the nighttime side of the Earth gradually turns toward
different parts of the sky. Hence, the particular stars that you see in the
night sky are different at different times of the year. This figure shows
which constellation is overhead at midnight local time—when the Sun is
on the opposite side of the Earth from your location—during different
months for observers at midnorthern latitudes (including the United
States). If you want to view the constellation Andromeda, the best time
of the year to do it is in late September, when Andromeda is nearly
overhead at midnight.

18. Constellations of the Zodiac
Note: ‘apparent’ position of Sun in the zodiac … could get same effect with Earth at center and Sun moving around it!

19. Winter & Summer The Sun is ‘in’ Capricorn on Dec 21
The Sun is ‘in’ Cancer on Jun 21
Figure Tropics and Circles
Four important latitudes on Earth are the Arctic
Circle (661⁄2° north latitude), Tropic of Cancer (231⁄2° north latitude),
Tropic of Capricorn (231⁄2° south latitude), and Antarctic Circle (661⁄2°
south latitude). These drawings show the significance of these latitudes
when the Sun is (a) at the winter solstice and (b) at the summer solstice.
An observer on Tropic of Capricorn would see Sun directly overhead at noon.
An observer on Tropic of Cancer would see Sun directly overhead at noon.

20. The Sun at 690 N. July 19 Figure 2-18 The Midnight Sun
This time-lapse photograph was taken
on July 19, 1985, at 69° north latitude in northeast Alaska.
At this latitude, the Sun is above the horizon continuously
(that is, it is circumpolar) from mid-May to the end of July.
(Doug Plummer/Science Photo Library)

21. Solar Energy in Summer & Winter
Figure Solar Energy in Summer and Winter
At different times of the year, sunlight strikes the ground at
different angles. (a) In summer, sunlight is
concentrated and the days are also longer, which
further increases the heating. (b) In winter the
sunlight is less concentrated, the days are short, and
little heating of the ground takes place. This
accounts for the low temperatures in winter.

22. Where is the Sun on the Celestial Sphere?
Tropic of Cancer
Summer Solstice
Autumnal Equinox
Equator
Vernal Equinox
Winter Solstice
Tropic of Capricorn

23. Figure 2-16 The Sun’s Daily Path Across the Sky
This drawing shows the apparent path of the Sun during the course of a day on four different dates. Like Figure 2-10, this drawing is for an observer at 35° north latitude.
In the winter, Sun rises in SE and sets in SW. It never rises higher than 250 above horizon (SLC).
In Spring and Fall, Sun rises in E and sets in W. It rises about 48 ½ 0 above horizon (SLC).
In the Summer, Sun rises in NE and sets in NW. It rises about 720 above horizon (SLC).

24. A ‘Geocentric’ Model
23 ½ 0
Figure The Ecliptic, Equinoxes, and Solstices
This illustration of the celestial
sphere is similar to Figure 2-14b, but is drawn with the north celestial
pole at the top and the celestial equator running through the middle. The
ecliptic is inclined to the celestial equator by 231⁄2° because of the tilt of
the Earth’s axis of rotation. It intersects the celestial equator at two
points, called equinoxes. The northernmost point on the ecliptic is the
summer solstice, and the southernmost point is the winter solstice. The
Sun is shown in its approximate position for August 1.

25. A ‘Heliocentric’ Model
Figure The Seasons
The Earth’s axis of rotation is inclined 231⁄2° away from the
perpendicular to the plane of the Earth’s orbit. The north pole is aimed at
the north celestial pole, near the star Polaris. The Earth maintains this
orientation as it orbits the Sun. Consequently, the amount of solar
illumination and the number of daylight hours at any location on Earth
vary in a regular pattern throughout the year. This is the origin of the
seasons.

26. Question
Which hypothesis below ‘explains’ why Earth experiences 4 seasons?
There are 2: (1) The fast rotating Earth slowly orbits the Sun in a plane that is tipped by 23.5o to its equatorial plane (2) The Sun moves slowly CCW along a plane that is tipped by 23.5o to the equator of the fast rotating celestial sphere.
The Sun and the Earth lie in the same plane.
The Earth is further away from the Sun in the winter and closer in the summer.
The Sun is brighter in the summer than it is in the winter.
The Earth’s orbit is perfectly circular.

27. Precession of a Top Figure 2-19 Precession
As the Earth precesses, its axis of rotation slowly traces out a circle in the sky, like the shifting axis of a spinning top.

28. Precession of Earth’s Axis
Figure Precession
Because the Earth’s rotation axis is tilted, the gravitational
pull of the Moon and the Sun on the Earth’s equatorial bulge together
cause the Earth to precess.

29. Projection of Earth’s Axis Onto Celestial Sphere
Figure Precession and the Path of the North Celestial Pole
As the Earth precesses, the north celestial pole slowly traces out a circle among the
northern constellations. At present, the north celestial pole is near the
moderately bright star Polaris, which serves as the North Star. Twelve
thousand years from now the bright star Vega will be the North Star.

30. Precession of the Equinoxes
The Vernal Equinox moves slowly through the zodiac in step with the precession of Earth’s axis

31. Measuring Time Solar day:
Time between 2 ‘transits’ of the local meridian by the Sun.
Figure The Meridian
The meridian is a circle on the celestial sphere that passes
through the observer’s zenith (the point directly overhead) and the north
and south points on the observer’s horizon. The passing of celestial
objects across the meridian can be used to measure time. The upper
meridian is the part above the horizon, and the lower meridian (not
shown) is the part below the horizon.
Sidereal day:
Time between 2 ‘transits’ of the local meridian by the Vernal Equinox.

32. Time Zones ~ 150 Intervals in Longitude
Figure Time Zones in North America
For convenience, the Earth is divided
into 24 time zones, generally centered on 15° intervals of longitude
around the globe. There are four time zones across the continental United
States, making for a 3-hour time difference between New York and
California.

33. Sidereal Time 10 more (3610) for a solar day ~ 24 hr.
Time to rotate 10 more is ~ 4 min.
Box 2-2 Sidereal Time (a) A month’s motion of the Earth along its orbit
If you want to observe a particular object in the heavens, the
ideal time to do so is when the object is high in the sky, on
or close to the upper meridian. This minimizes the distorting
effects of the Earth’s atmosphere, which increase as you view
closer to the horizon. For astronomers who study the Sun, this
means making observations at local noon, which is not too different
from noon as determined using mean solar time. For astronomers
who observe planets, stars, or galaxies, however,
the optimum time to observe depends on the particular object
to be studied. The problem is this: Given the location of a
given object on the celestial sphere, when will that object be
on the upper meridian?
To answer this question, astronomers use sidereal time
rather than solar time. It is different from the time on your
wristwatch. In fact, a sidereal clock and an ordinary clock even
tick at different rates, because they are based on different astronomical
objects. Ordinary clocks are related to the position
of the Sun, while sidereal clocks are based on the position of
the vernal equinox, the location from which right ascension is
measured. (See Box 2-1 for a discussion of right ascension.)
Regardless of where the Sun is, midnight sidereal time at
your location is defined to be when the vernal equinox crosses
your upper meridian. (Like solar time, sidereal time depends
on where you are on Earth.) A sidereal day is the time between
two successive upper meridian passages of the vernal equinox.
By contrast, an apparent solar day is the time between two successive
upper meridian crossings of the Sun. The illustration
shows why these two kinds of day are not equal. Because the
Earth orbits the Sun, the Earth must make one complete rotation
plus about 1° to get from one local solar noon to the next.
This extra 1° of rotation corresponds to 4 minutes of time,
which is the amount by which a solar day exceeds a sidereal
day. To be precise:
1 sidereal day 23h 56m 4.091s
where the hours, minutes, and seconds are in mean solar time.
Sidereal Day is time to rotate 3610 or 23 hr 56 min.