1. More than X marks the spot
Coordinate Systems
More than X marks the spot

2. Coordinate Systems Global – for the Earth Local – applies to you
Celestial (Equatorial or Polar)
- for the sky
Sky Movement
Daily
Yearly
26,000 years .

3. Altitude and Azimuth Points of reference
Zenith: Straight overhead
Meridian: Line from north to south, passing through the overhead zenith
North: That point on horizon toward the North Pole.

4. Local Coordinate System Altitude and Azimuth (Alt-Az) Altitude
Angle from the horizon upward.
Measured in degrees from 0 degrees at the horizon to 90 degrees straight overhead at the Zenith.
90º
45º 0º

5. Altitude and Azimuth Azimuth
Angle measured clockwise from north.
Measured in degrees, from:
0o north,
90o east,
180o south,
270o west,
back past 359o, to 0o north again

6. Altitude and Azimuth What can you use it for?
Use it for casual references to objects in the sky (e.g. announcements of comet positions).
Use it with alt-az telescope mounts which have digital setting circles that read in alt-az:

7. Celestial Coordinate System Right Ascension and Declination
Terms
Celestial sphere: an imaginary sphere, with Earth at the center, having an infinitely large radius.
All stars and celestial objects are projected onto the inside surface of this great sphere.

8. Right Ascension and Declination
North celestial pole (NCP) and south celestial pole (SCP): where Earth’s polar axis, if extended to the celestial sphere, would intersect.
Celestial equator: a plane from Earth outward to a line around the celestial sphere halfway between the NCP and the SCP.
Ecliptic: the plane in which the Earth orbits the Sun, or the Sun appears to orbit the Earth.

9. Right Ascension and Declination
Similar to latitude.
Measured in degrees, perpendicular to the celestial equator (north-south direction).
North is positive, south is negative.

10. Right Ascension and Declination
Similar to longitude.
Measured in hours, minutes and seconds around the sky, measured parallel to equator, eastward.
From 0 hr 0 min 0 sec to 23 hr 59 min … sec.

11. Right Ascension and Declination Where is the celestial Prime Meridian located and why?
Located at an important spot, where celestial equator and ecliptic cross.
This is where Spring is defined to begin.
Why? In order to keep the calendar consistent with the seasons.

12. Right Ascension and Declination Where is the celestial Prime Meridian located and why?
Greenwich Royal Observatory
Prime meridian appears in foreground.
Main instrument is a transit telescope
It moves only in altitude.
Installed by George Airy in 1850.
The foreground Prime Meridian seems to be enshrined in fluorescent (or neon) tubes, with brass frame on either side.

13. Closer view of the Observatory, with Airy’s transit scope.

14. Right Ascension and Declination How can you use it?
Knowing the coordinates of an object you want to see, from a star catalogue…
Observer’s Handbook star table.

15. Right Ascension and Declination How can you use it?
Locate the object at the coordinates on the star chart…
From Will Tirion’s Sky

16. Right Ascension and Declination How can you use it?
From the star chart, pick out recognizable constellation or star patterns in the area, and pick out marker stars to the object.

17. Right Ascension and Declination How can you use it?
Now find a similar star pattern in the sky…
Note conspicuous three stars in a row in lower right. These are Orion’s Belt. After that, locate Betelgeuse and Bellatrix (the shoulder stars).

18. Right Ascension and Declination How can you use it?
And follow the imaginary lines you mentally drew on the star chart.

19. The declination of my zenith is equal to my latitude!
MPHMPHMPH
Meridian
NORTH
SOUTH
EAST
WEST
Zenith
Polaris (NCP)
The declination of my zenith is equal to my latitude!

20. Declination and location
Declination of zenith = latitude
AND…
Latitude = angle of Polaris above northern horizon

21. Sky Movement - Daily Solar Day & Sidereal Day
In a 24 hour solar day, last night’s star arrives at tonight’s meridian 3 min 55.9 seconds earlier than last night.
Stars cross the meridian 3 m 55.9 s earlier each night.
Sidereal day is 23 hr 56 min 4.1 sec long.

22. Sky Movement - Daily Solar Day & Sidereal Day

23. Sky Movement - Annual
As a consequence, each night the stars appear about 59 arc-min (~10) farther west at the same time as the night before.
Over one year, the entire sky will completely slip past.
Therefore, what is up tonight depends not only on what time of night it is, but on which night of the year it is. Last month’s stars are 2 hours farther west.

24. Sky Movement - Annual 9:00 pm, May 15, 2005
Better viewed at about 150% magnification, so the meridian (red, dotted, vertical line) is clearly visible. Note which stars are on the meridian this month.

25. Sky Movement - Annual 9:00 pm, June 15, 2005
Note how the stars that were on the meridian at the same time last month, have moved 2 hrs of RA to the west.

26. Sky Movement - Annual 9:00 pm, July 15, 2005
Two months later, have moved about 4 hours further west.

27. Sky Movement - Annual Manage both: time of night, and
night of the year
with a Planisphere.

28. Using your star wheel Set date and time Hold overhead Align horizons
Read constellations

29. Sky Movement – 26,000 years
The Roman calendar, as established by Julius Caesar, had problems:
Started at Winter Solstice on January 1, 45 BC.
Year was 365 ¼ days long, with leap year corrections. Decimals had not been invented yet;
Year is actually … days.
So, after a few centuries, people noticed the seasons (equinoxes) were moving slowly in relationship to their calendar.

30. Precession of the Equinoxes
Earth’s drifting polar axis causes the North Celestial Pole to move in a circle around the sky, once every 26,000 years.
Thuban was the pole star when the Great Pyramid was built in Egypt.

31. Summer Solstice, 400 BC
In order to find the Sun in the constellation Cancer, one must go back (by reverse precession) about 2400 years. This is evidence that the expression “Tropic of Cancer” is about 2400 years old.

32. Summer Solstice, 2005 AD
TheSky, Summer Notice the Sun on it’s path is farthest above the equator (blue horizontal line that passes through Orion’s belt). On this day (6/21/06 in lower right of screen) at 1:46 PM, the Sun was at it’s farthest north. The Sun’s position is between the constellations of Gemini and Taurus. After this the Sun appears to be going around the Earth in a progressively southern direction.

33. Spring Equinox, 2005 AD

34. Spring Equinox, 2600 AD
Going forward in time 600 years finds the Sun on the Vernal Equinox (first day of Spring), to be entering the constellation Aquarius. This is the time of the song title “Dawning of the Age of Aquarius”

35. Leap Years
Gregorian solar year: 365d 5h 48m 20s; still has s/SY error.
Summer solstice moves later on calendar each year. Slightly over-corrected each leap year.
Cumulative annual error is corrected each century by not having a leap year, unless that year is divisible by 400 (1600 and 2000 were leap years).
The Gregorian Calendar started in 1582 AD, as Pope Gregory tried to have the calendar and seasons reconciled.

36. Why bother with such tedious accuracy ?
Nearly all major advances in astronomy occurred when observation did not agree with prediction, and people needed a better theory for better predictions.
Astronomy advanced as instruments became precise enough to measure the flaw in prevailing theory.
By the middle of the 19th Century we could predict the movement of stars and planets within a fraction of a second, except….
I have gone to editorializing….

37. What does this lead up to?
The perihelion of Mercury (point closest to the Sun) was observed to be advancing in longitude by about half a minute of arc per century faster than prevailing celestial mechanics predicted.
Which is where Newtonian Theory ends, and Relativity Theory begins.
Scientific advancement is a journey, not a destination. Our trip continues with each new discovery.
Do the math. Imagine a US quarter, seen at a distance of 3.25 miles. Now imagine an ant, walking slowly from the center of that quarter to the edge, and taking a hundred years to do it. Now imagine the quarter is not there for comparison, just the ant hanging in space, moving a ½ a quarter diameter per century.

38. Web Stuff

39. Credits: Books All About Telescopes, Edmund Scientific, 1975
Calendar, David Ewing Duncan, 1998
Cambridge Illustrated History of Astronomy, Michael Hoskins, ed., 1997
Instruction Manual, Meade Instruments Corp, 1996
Observer’s Handbook 2005, RASC, 2004
Sky , Will Tirion, 1981
The Practical Astronomer, Colin a Ronan, 1984

40. Credits: Web sites
For magnetic declination for any latitude and longitude:
For seasons and ecliptic tilt:
For history of Royal Observatory at Greenwich:
Please visit these web sites. This is just the tip of the iceberg for astronomical information that is out there. Google any astronomical word, name or expression, and you will receive more leads than you can follow on a cloudy evening.