Physics 55Friday, September 16, 2005

1. Finish Chapters 2 and S12. Celestial coordinates: RA and dec

Announcements• Change in final grade weighting: 1. increase from 10% to 15% for observation sessions 2. decrease from 25% to 20% for quizzes Sign up for observation night via Blackboard by Sunday midnight 9/18 or get 10% observation grade deduction!

• Viewing sessions will start up again after Sept 25 because of full Moon.

• One student needs a project partner: please send me an email if you are working on your own or with more than one partner.

• Will be out of town next Friday, Sept 23. Physics Professor Ronen Plesser will give a guest lecture on string theory and will answer any questions that you might have. You will need to hand in a one-page summary of the lecture by the end of the lecture!

• Read course announcements as usual for rest of news: what to read, events in the area, astronomical news.

Do the Astronomy Place Tutorials!

Access from my web page, you will need login and password from your text.

Tutorials are quite good, supplement the lectures and text well.

http://www.aw-bc.com / astronomyplace/

Future Culture Shocks

Overwhelming choice of class was that discovery of intelligent alien life would be next biggest shock. Many students thought this likely.

A few students argued that the big shocks are over: the human race is too aware now of the possibilities for its assumptions to be falsified.

Other shocks?- Definitive proof that a deity or creator exists?- Electronic expansion of brain via chips and internet: perfect memory, instant access to all knowledge, scalable computing abilities? - Computers much smarter than any human?

Extra credit book reports: “The End of Science” by John Horgan “Just Six Numbers” by Martin Rees

Is the World Ready for Aliens?

Celestial Coordinates: declination dec and right ascension RA

The spring equinox (also called vernal equinox) is where the ecliptic crosses the celestial equator while moving northerly. It plays the role of the prime meridian in terms of defining zero RA.

Note: RA is measured in sidereal time from 0h to 24h, not degrees, since the celestial sphere turns once every sidereal day of 24 hours.

1 hr RA = 15o

Sun’s Celestial Coordinates

Winter solsticeRA 18h dec -23.5o

Summer solstice:RA ? dec ?

Fall equinoxRA? dec?

Polaris:RA? dec?

Warning: because Earth’s orbit is elliptical, Sun’s progression along ecliptic is not at constant speed (RA does not change at constant rate), is a bit faster in winter and slower in summer.

Axis of the Earth Precesses So Celestial Coordinates Need Epoch Time Specified

Slow precession due to weak pull of Moon and of Sun on Earth’s “waist” or bulge causes the North Celestial Pole to drift slowly in a circle over about 26,000 years. This is one of many astronomical consequences of the conservation of angular momentum.

This is why Polaris will not always be the “pole star”, other north stars have included Vega and Thuban. Skygazer has nice demo: go to menu “Explore” then select menu item “Wobble of the Earth…” Look also at Chart/Precession Epoch in Skygazer. Look also at File/Open Settings…/Basics/Precession of the Equinoxes.vgr

What is right ascension good for?

1. All objects with same RA cross your meridian at the same time.

2. Corollary: all objects with RA close to that of Sun on given day will be close in sky to the Sun and will be hard to see because of brightness of Sun in Earth’s atmosphere.

3. RA of an object tells you how long in sidereal time an object will appear on your meridian after the spring equinox has appeared on your meridian: you can predict when an object appears.

4. Difference in RAs for two objects is amount of sidereal time that has to pass for one object to appear on your meridian after the first object has crossed your meridian.

Use Skygazer to connect celestial coordinates to your sky

How to Relate The Three Coordinate Systems:Local Sky, Longitude/Latitude, Celestial Coordinates

I will explain this important diagram and following slides at the white board.

This diagram mainly links the “up and down” coordinates: latitude on Earth, altitude in local sky, and dec of celestial sphere.

The right-left coordinates (longitude, direction, RA) are more difficult to relate since one needs a sidereal clock.

Local Sky, Earth, Celestial Coordinates:Picture on Whiteboard

Deductions:1. Altitude of north celestial pole is latitude of position

on Earth. (Altitude of Polaris gives latitude to plus or minus 1o accuracy.)

2. Altitude of celestial equator along meridian is 90o–latitude.3. Only stars with declinations greater than –(90o–latitude) can be observed in local sky.4. Only stars with declinations greater than 90o – latitude are circumpolar (never setting).

PRS Question

Star RA Dec Constellation

1. Alpha Centauri 14h 36m -60o38’ Centaurus

2. Arcturus 14h 13m +19o27’ Bootes

3. Canopus 6h 23m -52o42’ Carina

4. Sirius 6h 43m -16o39’ Canis Major

5. Vega 18h 35m 38o44’ Lyra

Which star is closest to the celestial equator?

PRS Question

Star RA Dec Constellation

1. Alpha Centauri 14h 36m -60o38’ Centaurus

2. Arcturus 14h 13m +19o27’ Bootes

3. Canopus 6h 23m -52o42’ Carina

4. Sirius 6h 43m -16o39’ Canis Major

5. Vega 18h 35m 38o44’ Lyra

Which star is closest to the fall equinox?

PRS Question

Star RA Dec Constellation

1. Alpha Centauri 14h 36m -60o38’ Centaurus

2. Arcturus 14h 13m +19o27’ Bootes

3. Canopus 6h 23m -52o42’ Carina

4. Sirius 6h 43m -16o39’ Canis Major

5. Vega 18h 35m 38o44’ Lyra

Which star is never visible from Durham?

PRS Question

Star RA Dec Constellation

1. Alpha Centauri 14h 36m -60o38’ Centaurus

2. Arcturus 14h 13m +19o27’ Bootes

3. Canopus 6h 23m -52o42’ Carina

4. Sirius 6h 43m -16o39’ Canis Major

5. Vega 18h 35m 38o44’ Lyra

Which star will pass closest to your zenith in Durham?

PRS Question

Star RA Dec Constellation

1. Alpha Centauri 14h 36m -60o38’ Centaurus

2. Arcturus 14h 13m +19o27’ Bootes

3. Canopus 6h 23m -52o42’ Carina

4. Sirius 6h 43m -16o39’ Canis Major

5. Vega 18h 35m 38o44’ Lyra

In Quito, Ecuador, which star will cross your meridian at about midnight in late June?