Today’s Lecture
• How to find astronomical objects
– Constellations, coordinates, precession
• Motion of stars on the sky
• First homework is due at the start of
class on Monday.
How does one locate an object on
the night sky?
1. By drawing imaginary patterns on the sky
(the constellations) and then locating the
object relative to the stars in the
constellations.
2. By drawing an imaginary coordinate
system on the sky, then specifying the
objects coordinates.
Eighty-eight constellations cover the
entire sky.
• 6000 stars visible to
unaided eye (only half
are above the horizon).
• 88 semi-rectangular
groups of stars called
constellations.
• Some stars in the
constellations are quite
close while others are
very far away.
Finding M51
Finding M51
Finding M51
We use angles to denote the positions and
apparent sizes of objects in the sky.
Your hand at arm’s length is about 10 degrees wide
Your thumb at arm’s length is about 2 degrees wide
Coordinate system
Coordinates are
Latitude = degrees
North or South of the
equator
Longitude = degrees
East or West of the
“Prime meridian”
Prime meridian is
historically defined as
longitude of the Royal
Observatory in
Greenwich, England
Sky coordinate system
Introduce the
‘celestial sphere’
This is an imaginary
sphere drawn in space
with the earth at its
center.
We align the sphere
with the Earth.
Coordinates are:
Declination = degrees
North or South of the
equator.
Right ascension =
hours (or degrees) East
of the “Vernal equinox”.
Vernal equinox is
defined as the position
of the Sun on the first
day of spring. Note it is
a point on the sky, not
the earth.
Precession
• If you spin a top, its very hard to get it to
spin exactly straight – usually it wobbles
around in a circle
• The spinning Earth wobbles in exactly the
same way – this is called precession
Precession of the Earth
Precession causes celestial coordinates to change slowly with time.
When observing, one must have coordinates for the correct epoch.
Motion
of stars
on the
sky
The rotation of the Earth causes the stars to
appear to move on the sky.
Angular Measure for Small
Angles
1º = 60 arcminutes = 60′
1′ = 60 arcseconds = 60″
e.g., On January 1, 2004, the planet Saturn
had an angular diameter of 19.7″ as viewed
from Earth.
Coordinates
• Right Ascension hours:minutes:seconds
– There are 24 hours in 360°.
• Declination degrees:arcminutes:arcsecond
– 1º = 60′, 1′ = 60″
• Polaris: RA = 02h31m49s, Dec = 89°15′51″ (J2000)
• Sirius: RA = 06:45:09, -16:42:58 (J2000)
• Sirius lies in Southern hemisphere, but is visible from Iowa.
• Stars visible from Iowa have Dec > 41.7° - 90° = -48.3°.
Hour angle
• Meridian is the half-circle running from one pole to the other through
a point directly overhead. During the night, a star will reach its highest
elevation above the horizon when it passes through the meridian
(‘transits’).
• Sidereal time is time kept relative to the stars (rather than the Sun).
Local sidereal time = 0h when a star with RA = 0h crosses the
meridian (like noon = Sun crossing the meridian).
• RA uses time units to make it easy to compute the position of a star on
the sky at any time of night.
• A star transits when the sidereal time equals its right ascension.
• Hour angle = Local sidereal time - right ascension = position of star
relative to meridian.
• RA determines what time of year an object is visible. On March 21,
objects with RA = 12h 00m transit at midnight.
• Best to observe objects when they are high in the sky to minimize the
amount of air you need to look through.
Coordinates
• Which objects would be reasonable choices for 29:62
observing projects? (All in J2000)
•
•
•
•
12 29 07, 02 03 09 (3C273)
05 23 35, -69 45 22 (LMC)
00 42 44, 41 16 08 (M31)
02 31 49, 89 15 51 (?)
• Object Seasonal Observability (Rigel is near Kitt Peak)
• Coordinate converter
Review Questions
• What is the angular size of the Moon? How
does it compare to the angular size of the Sun?
• Why are the coordinates (RA, Dec) for a star
measured in 1950 different from the coordinates
measured for the same star in 2000, even if the
star is not moving in space?
• Would it be prudent to do an observing project
on M82 for this class?