Transcript Sidereal Time and Celestial Coordinates

Comet Machholz passes the
Pleiades
Comet Machholz C/2004 Q2
• Discovered byDonald Machholz, Jr. on
August 27, 2004
• Period of about 120,000 years
• Just up to naked eye visibility now, but
much easier to see in binoculars
Comet Machholz’s path through the
sky
By the end of this week, the moon will be brightening
in the evening sky, making observing more difficult
Until moonset.
Comet Machholz’s two tails
Sidereal Time and Celestial
Coordinates
AST 208 Web Page
• The AST208 web page now has the
syllabus and the powerpoint file from the
first lecture:
http://www.pa.msu.edu/courses/AST208/
Celestial Coordinates
• Altitude and azimuth
Right ascension and declination
Declination
The Daily Motion
daily circles --- CCW looking north, CW looking south
Circumpolar stars
Which stars are circumpolar?
• The altitude of the North Celestial Pole is
equal to our latitude, about 43 degrees.
• Only those stars within 43 degrees of the
NCP are seen as circumpolar at our
location
• So stars with a declination greater than
90 - 43 = 47 degrees are circumpolar for
us
• Which of the following stars are
circumpolar as seen from a latitude of 60
degrees north?
– Polaris declination = 89 degrees
– Sirius declination = -17 degrees
– Vega declination = 39 degrees
Some stars never rise above our
horizon
• A star directly overhead has a declination
equal to your latitude. A star that just
manages to appear above your southern
horizon will be 90 degrees further south.
• A star with a declination below -47
degrees will never rise above our horizon
Rules
• For an observer at latitude x north
– Circumpolar: stars of dec > 90-x
– Never seen: stars of dec < -(90 –x)
– All of the stars with inbetween declinations
are sometimes above our horizon and
sometimes below it
If you are at the North Pole
• Which stars are circumpolar?
• Which stars would you never see?
If you are at the equator
• Which stars are circumpolar?
• Which stars do you never see?
Local Skies
• Lines of constant declination cross the
sky at different altitudes, depending on
your location on Earth.
– declination line = your latitude goes
through your zenith
– the altitude of the N or S celestial pole =
your latitude
Celestial Coordinates
•
•
Movie. Click to play.
lines of Right Ascension & Declination
lines of constant R.A. continually move
in the sky as Earth rotates
Hour Angle
Big sunspot group
Huygens probe landed on Saturn’s
moon Titan
• First data expected
later today if all goes
well
Solar vs. Sidereal Day
•
•
•
Sidereal day – time it takes a star at the meridian to return to the meridian.
– 23 hours 56 min 4 sec
Solar day – time it takes the Sun at meridian (noon) to return to the meridian.
– noon to noon or 24 hours
Why the 4-minute difference?
– as it rotates, the Earth also orbits the Sun
– Earth must rotate an extra degree (4 min) each day…
– for any observer on Earth to be at noon again
What is the LST?
• LST = local sidereal time
• LST = 0 when the Vernal equinox is
transiting the meridian
• LST = 0 at local midnight at the autumnal
equinox
– LST is 2 hours later at midnight for every
month that goes by since the autumnal
equinox
What is the LST at local midnight
tonight?
• LST = 0 + 2 X (3.7 months) = 7.4 hours
7.4 hours = 7 hours 24 min
• At midnight eastern standard time
LST = 7 h 24 m – 37 minutes = 6 h 47 min
A star with right ascension 6h 47 min would be
on the meridian at that time
What is the LST at 8pm?
• LST = 6 h 47 min – 4 = 2h 47 min
Annual Motion of the Sun
• The R.A. of the Sun…
– increases about 2 hours per month
• The Declination of the Sun…
– varies between –23º and +23º
The Cause of the Seasons
Review questions (open notes)
1. At local midnight on April 1, 2005, what will be the approximate LST?
2. You are on a ship at latitude 10 degrees north.
a. Stars of what declination are circumpolar for you?
b. Stars of what declination never rise above your horizon?
3. At the summer solstice, around June 21, what is the declination of the
sun?
4. At the summer solstice what is the altitude of the sun at local noon as
seen from East Lansing (latitude about 43 degrees)?