PHYS216 Practical Astrophysics Lecture 1 Observing the Sky from Earth

1. PHYS216 Practical AstrophysicsLecture 1Observing the Sky from Earth Module Leader: Dr Matt Darnley Astrophysics Research Institute Liverpool John Moores University M.J.Darnley @ ljmu.ac.uk Course Lecturer: Dr Chris Davis Liverpool Telescope/ARI Liverpool John Moores University C.J.Davis @ ljmu.ac.uk

2. Liverpool Telescope

3. The LT is part of theObservatoriodel Roque de • Los Muchachos (ORM) which is located on the • summit of La Palma, Altitude 2,363 m

6. Liverpool Telescope http://telescope.livjm.ac.uk/ http://www.facebook.com/liverpooltelescope

7. Notes online All course materials available on Vital These Powerpoint slides: • http://www.astro.ljmu.ac.uk/~cjd/Teaching/ Other good resources online: • http://star-www.st-and.ac.uk/~fv/webnotes/ • http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/instruments/phy217_inst_course.html

8. Great Circles and Small Circles • Great Circle • The intersection of a plane containing the centre of a sphere and its surface, e.g. ABCD and BEDF. • P and Q are poles of plane ABCD. • Small Circle • A circle which does not include the centre of the sphere, e.g. WXYZ.

9. On the Earth… • All lines of Longitude are great circles • All lines of Latitude are small circles except the equator. Q. Is the tropic of Cancer a small or a great circle? Lines of equal LATITUDE Lines of equal LONGITUDE

10. Spherical Trigonometry Spherical triangle Formed from three arcs of Great Circles. Distances (a; b; c) are measured as angles. Sum of the 3 angles A + B + C > 180o(this is non-Euclidean geometry appropriate to curved space). • e.g. if • arc AB is part of the Earth's equator • arc CB is the Greenwich Meridian (long=0o) • arc CA is longitude=90o • then A = B = C = 90o • so A + B + C = 270o in this case. • As the triangle gets small relative to the size of the sphere then • A + B + C -> 180o

11. Spherical sine rule: Spherical cosine rule: How do these rules relate to Euclidean (plane) geometry? If the length of the sides are very small (compared to the radius of the sphere), then sin a ~ a etc. (small angle approximation). The spherical sine rule then becomes: The Euclidean cosine rule can also be recovered by the same method.

12. Celestial Sphere • Celestial Meridian – a great circle which passes through Zenith and the North & South celestial poles. It is perpendicular to the horizon. • If you stand facing North, the meridian is a line that passes from north on the horizon, directly over your head, to south on the horizon behind you

13. Celestial Sphere As the earth rotates, stars (like the sun) rise in the east, pass over the meridian (transit), and set in the west. The hour angle tells you how long it will be before the star transits (or how much time has passed since it transited!) • Hour Angle - angle between a star's current position and the meridian (measured WESTWARD in hours, where 1 hour is equivalent to 15 degrees – because 24 hours = 360 degrees). MERIDIAN An object transits or culminates when passing through the meridian. It has an HA = 0 hr when culminating. Its HA then increases as it moves towards the west. AT HA = 23h it is just one hour short of culminating again. HA ~ 3 hr S E W

14. Celestial Sphere MERIDIAN Facing north it’s t’other way around! The stars rise on your right and move towards the left… HA ~ 21 hr N W E

15. Celestial Sphere • Celestial Equator- Projection of the Earth's equator out onto the Celestial Sphere. d • Celestial Poles – we have two of ‘em, a North and South pole! • Declination- angular distance of a star above the Celestial Equator. Analogous to LATITUDE, hence the Celestial Equator comprises all points at zero Declination.

16. Observing the sky from the surface of the rotating Earth Horizon – you can’t see below this, so some stars are too far south to be observed from e.g. Europe. Zenith – directly overhead Altitude– angular height of star above the horizon Zenith distance/angle – angle between zenith and direction to star. Apparent direction of stars Altitude of the pole above the horizon = latitude of the observer, f (phi)

17. The Celestial Sphere Star X Path of Star X on the sky is a small circle (red) parallel to the celestial equator (blue). It rises in the East, transits the meridian, and sets in the West. Star X rises and sets where its small circle intersects the observer’s horizon (black). • General rule - for an observer in the NORTH: • If a northern hemisphere star’s small circle does not intersect the observer’s horizon, the star never SETS and the star is said to be circum-polar • If a southern hemisphere star’s small circle does not intersect the observer’s horizon, the star never RISES

18. Circumpolar Stars These objects (at high declination) never set, and depend on the observer's latitude f For a star that just grazes the horizon at its lowest elevation, declination d is given by: d = 90o – f i.e. if d > 90o - f then the star is circumpolar The Latitude of Liverpool is 53o N. Therefore stars with d > 37o never set! At the north pole, essentially all visible stars are circumpolar; at the equator, none are! At what declination do stars never rise in Liverpool?

19. Circumpolar Stars Upper transit (or culmination) Lower transit (often below the horizon) N E Looking North… Stars rise in the East and rotate in an anti-clockwise direction.

20. Skycam-A at the LT Stars rise and set like the Sun (well, most of them do…) Watch out for Perseid Meteors towards the end of the night! Skycam-A is mounted on the wall of the telescope enclosure pointing up at the roof…

21. Skycam-T and -Z at the Liverpool Telescope SkyCam-T : 21o Field of View SkyCam-Z : 1o Field of View For more Movies (three every night, in fact) Google Liverpool Telescope and check out the Night Reports link on the side-bar.

22. Star Trails The image on the left shows star trails above the Observatorio del Teide, Tenerife; the image on the right shows the night sky above UKIRT in Hawaii. Both telescopes are in the northern hemisphere. Q1. But which observatory is furthest north? Q2. Both pictures are facing North – what do the stars do if you look south? Q3. How could you calculate your latitude from these photos? Q4. How can you calculate the duration of the exposure in each photograph?