Chapter 1
Astronomy Today
7th Edition
Chaisson/McMillan
Charting the Heavens
Day 2
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
• 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
Earth’s Orbital Motion
• Daily cycle, noon
to noon, is
diurnal—solar day
• 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
Twelve constellations (some say thirteen) Sun moves through during the year are called
the zodiac; path is ecliptic
Sun signs
• Are based on which constellation the sun was
in 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 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
Seasons
• Turn to your elbow partner and discuss why
you think we have seasons>WHEN YOU THINK
YOU HAVE AN ANSWER < DRAW OR DESCRIBE
YOUR ANSWER ON YOUR WHITE BOARD> ALL
TABLES WILL BE REPORTING WHAT THEY
THINK SO BE PREPARED.
Ecliptic
• If the sun's path is
observed from the Earth's
reference frame, it
appears to move around
the Earth in a path which
is tilted with respect to
the spin axis at 23.5°.
• The angle of earth’s
axis=the plane of the
angle of the ecliptic to the
celestial equator
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
Seasons
• On the Equinox, the Earth experiences equal
amounts of day and night
• Summer solstice- the sun is at it’s highest
point
• Winter Solstice –the sun is at it’s lowest point
Local co-ordinate systems
• Based on the objects above the plane of the
horizon
– Altitude is the angle above the horizon
star
altitude
horizon
NORTH STAR HAS AN ALTITUDE OF 0° IF YOU ARE ON THE EQUATOR
THE ALTITUDE OF THE NORTH STAR=YOUR LATITUDE ON EARTH!!!
A ROUGH WAY TO ESTIMATE ALTITUDE
• PINKY =1°
• 3 FINGERS=3°
• FIST = 10°
Local Co-ordinate System
• Azimuth – starts with north a 0° and south is
180°
• Zenith is 90°
zenith
North Star
• Even though Polaris is currently the North star,
it doesn’t lie due North –and eventually will
move Vega will be our North Star, Why do you
think this is happening? Discuss with your
elbow partner, write down your thoughts on
your white board, be ready to defend them.
CIRCUMPOLAR STARS
• STARS THAT NEVER GO BELOW THE HORIZON
– CAN ALWAYS BE SEEN AT NIGHT
– http://upload.wikimedia.org/wikipedia/commons
/2/21/Zirkumpolar_ani.gif
The North Star
• Polaris, our current North Star is the 49th
brightest star in the night sky!!!!!
• To find it, locate the cup of the Big Dipper to
the handle of the Little dipper.
• It doesn’t appear to move in the night sky but
the other stars rotate around it
• North pole and Polaris are about 1° off from
one another
Earth’s Orbital Motion
Precession: rotation of Earth’s axis itself; makes one complete
circle in about 26,000 years
Precession
• Wobbles in a 26,000 year cycle
• Wobble is between Polaris, Vega and Thuban
• Changes the position of the Vernal Equinox
which will also change the co-ordinates of the
stars
SEASONAL CHANGES IN OUR NIGHT
SKY
• Summer triangle: Vega, Denab, Altair
• Winter: Orion, Sirius (Dog Star)
• The change occurs about 1°/night
1.4 Earth’s Orbital Motion
Time for Earth to orbit once around Sun, relative to fixed stars, is sidereal year
Tropical year follows seasons; sidereal year follows constellations—in 13,000 years July
and August will still be summer, but Orion will be a summer constellation