Chapter 1
Astronomy Today
7th Edition
Chaisson/McMillan
Charting the Heavens
Day 2
Going to the Stars
Road at Logan Pass,
Glacier National Park.
This picture shows
our view of the Milky
Way in the night sky.
Celestial Sphere
The celestial sphere:
Stars seem to be on the inner surface of a
sphere surrounding the Earth
They aren’t, but they can use two-dimensional
spherical coordinates (similar to latitude and
longitude) to locate sky objects
Celestial Coordinates
Right Ascension
• Like longitude
• Use units of time-hours
instead of degrees
• 0 hour is the vernal equinox
Declination
• Like latitude except use +/instead of north and south
Terms related to the Celestial Sphere
Terrestrial System
• South Pole
• North pole
• Equator
• Latitude
– 0° at the Equator
• Longitude
– 0° at the Prime Meridian
Celestial System
• South Celestial Pole
• North Celestial Pole
• Celestial Equator
• Declination
0° at celestial Equator
• Right Ascension
– 0 Hours at Vernal Equinox
Angular Measure: A way to describe the amount
of sky a celestial body takes up
• Full circle contains 360°
(degrees)
• Each degree contains 60′
(arc-minutes)
• Each arc-minute contains 60′′
(arc-seconds)
• Angular size of an object
depends on its actual size and
distance from viewer
1. What is apparent size?
2. What happened with the coin demonstration?
3. What is the equation used to figure angular diameters
With a partner work on Problems 1
through 6 on the worksheet
Earth’s Orbital Motion
• Daily cycle, noon
to noon, is
diurnal—solar day and is based on
the sun’s position.
• Stars aren’t in quite the same
place 24 hours later, though, due to
Earth’s rotation around Sun; when
they are once again in the same
place, one sidereal day has passed
Earth’s Orbital Motion
Seasonal changes to night sky are due to Earth’s motion around Sun
Earth’s Orbital Motion
The Twelve constellations (some say thirteen) that the Sun moves through during the
year are called the zodiac; The view of the night sky changes as Earth moves in its orbit
about the Sun. As drawn here, the night side of Earth faces a different set of
constellations at different times of the year. The 12 constellations named here make up
the astrological zodiac.
Turn to your
neighbor and
discuss what
Astrological
sign you have
been told you
are.
Now let’s took a look at Stellarium to
find your corrected astrological sign.
• Put your birth date in. Locate the sun, turn on
the constellations so you can see what sign
the sun was in on your birth date.
Sun signs
• Are based on which constellation
the sun was on the day of your birth
• Moon sign: which constellation is
the moon is at the time of your
birth
• Most astrological signs are
incorrectly shown because they
are based on your birth where the
sun was during Greek times.
Ophiucus
•
•
•
•
•
The thirteenth zodiac sign
Sun passes through Ophiucus’ foot
November 30-Dec. 18
He is the serpent bearer
also used as the medical symbol
1.4 Earth’s Orbital Motion
• Ecliptic is plane of Earth’s path around Sun; at 23.5° to
celestial equator
• Northernmost point of path (above celestial equator) is
summer solstice; southernmost is winter solstice; points
where path crosses celestial equator are vernal and
autumnal equinoxes
• Combination of
day length and
sunlight angle
gives seasons
• Time from one
vernal equinox to
next is tropical
year
1.4 Earth’s Orbital Motion
Precession: rotation of Earth’s axis itself;
makes one complete circle in about 26,000
years
The Measurement of Distance
Triangulation:
Measure baseline
and angles, can
calculate distance
The Measurement of Distance
Parallax: Similar to
triangulation, but look at
apparent motion of object
against distant background
from two vantage points
The Measurement of Distance
Measuring Earth’s radius:
Done by Eratosthenes
about 2300 years ago;
noticed that when Sun
was directly overhead in
one city,
it was at an angle in
Another due to the
earth’s curvature.
Measuring that angle
and the distance
between the cities gives
the radius.
Measuring Distances with Geometry
Converting angular diameter and distance into size