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 brighteningin the evening sky, making observing more difficultUntil 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• lines of Right Ascension & Declination• lines of constant R.A. continually move

in the sky as Earth rotates

Movie. Click to play.

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 hours7.4 hours = 7 hours 24 min

• At midnight eastern standard timeLST = 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)?