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### Precession, nutation, aberration, refraction, parallax, etc ... The position of Dubhe (a UMa), the last star in the bowl of the Big Dipper, can be given as: ... – PowerPoint PPT presentation

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Title: Sources:

1
Astronomical Observing Techniques Coordinate
Systems Lecturer Nigel Douglas
• Sources
• 1. Adler Planetarium and Astronomy Museum,
Chicago
• 2. Hartmut Frommert www.seds.org
• 3. Juergen Giesen www.geoastro.de
• 4. S.W.Digel (SLAC)

2
• The Horizon System
• Celestial Sphere
• Equatorial System
• Distances on the Celestial Sphere
• Ecliptical Coordinate system
• Galactic Coordinate system
• Precession, nutation, aberration, refraction,
parallax, etc

3
The Horizon System(a.k.a. Alt-Az system)
• Observer-centered.
• Measure Azimuth from N through
East (0-360 deg)

4
The Horizon System
• Altitude is measured in decimal degrees, up from
• Also called Elevation.

5
The Horizon System
• Your zenith is the point directly above your
head, at an altitude of 90º.
at an altitude of -90º.

6
The Horizon System
• The zenith angle of a point on the sky is its
angular distance from the zenith.
• Zenith angle and altitude are complementary
angles. (They sum to 90º.)

7
The Horizon System
• Quality of astronomical observations gets poorer
as you look closer to the horizon, because youre
looking through more atmosphere.

8
The Horizon System
• When you look straight up, we say that your
observation has an airmass of 1.

9
The Horizon System
• The airmass for an observation at zenith angle z
is given by sec(z).
• sec(45º) 1.4 sec(60º) 2

z
10
The Horizon System
• Your meridian is an imaginary line drawn across
the sky, starting due North of you, passing
through your zenith, and ending due South of you.

11
The Horizon System
• A celestial object is said to transit or
culminate when it crosses your meridian.

12
The Horizon System
• Most celestial objects are at their highest
altitude (lowest airmass) of the night as they
transit.
• This is how RA used to be measured (transit
telescope or meridian circle)
• Kitchin p376

13
The Horizon System
• Cant be used to give unique coordinates to
astronomical objects - changes with time and
with position of observer.

14
The Celestial Sphere
• It is convenient to talk about a celestial
sphere, upon the inside of which all of the fixed
stars appear to be painted.

15
The Celestial Sphere
• The celestial sphere appears to rotate once about
the north celestial pole in 23 hrs, 56 min.
• This sidereal day is different from the 24-hr
solar day because the Earth orbits the Sun.

16
The Equatorial System
• Project the Earths equator and poles onto the
celestial sphere.
• A common astronomical coordinate system for all
observers on earth!

17
The Equatorial System
• Declination is measured north or south from the
celestial equator, toward the poles.
• NCP has dec 90º
• SCP has dec -90º
• Typically quoted in º / / .

18
The Equatorial System
• Right Ascension is measured east along the
celestial equator.
• The reference point for RA 0 is the Suns
position on the celestial sphere during the
vernal (spring) equinox.

19
Vernal equinox, Mar 21, is the first day of NH
spring.
www.crbond.com
20
The Equatorial System
• Right Ascension is measured east along the
celestial equator.
• The reference point for RA 0 is the Suns
position on the celestial sphere during the
vernal (spring) equinox.

21
The Equatorial System
• Right Ascension is not measured in degrees, but
in units of time!
• It is in fact the extra time that a star with
that RA would take to reach the meridian through
the vernal equinox after the sun.
• 1h 60m of RA
• 1m 60s of RA

22
The Equatorial System
• Converting the units of R.A. into true angular
units...
• 1h of R.A. 15º
• 1m of R.A. 15
• 1s of R.A. 15

Except that they arent !!!
23
The Equatorial System
• The position of Dubhe (a UMa), the last star in
the bowl of the Big Dipper, can be given as
• 11h 03m 43.5s, 61º 45 03
• or
• 110343.5, 614503
• or simply
• 11 03 43.5, 61 45 03.

Why more digits for RA?
24
Distances on the Sky
• For celestial objects within about 10 of each
other (e.g., in the same telescope field of
view), the angle d between them is given by
• d2 (Dra ? cos(decave))2 (Ddec)2
• Here, the units of R.A. and Dec must be degrees.
(Convert first.)

25
Distances on the Sky
• For further-separated objects this equation
doesnt work, for the same reason that Muslims in
New York pray towards the northeast...
• The shortest distance between two points on a
sphere is a great circle!

26
Distances on the Sky
• For further-separated objects, the correct
distance equation is given by
• cos d sin dec1 sin dec2 cos dec1 cos dec2 cos
Dra

27
Equatorial coordinates summary
• RA, Dec or a, d
• natural choice for astronomy from earth
• one number in catalogs
• you can tell right away whether a given position
will rise, how high it will reach, and what time
of year it will be up at night.
• N.B. Epoch must always be specified -
• Precession period 26,000 yr 20/yr

28
Galactic coordinates
• b and l (galactic latitude and longitude)
• Natural for middle astronomy
• Relevant for extragalactic observations
• (foreground emission/obscuration)
• Plane of the Milky Way
• traces Galactic Equator
• (0,0) is direction to the Galactic center
• (180,0) is the anticenter

Powell
29
Galactic coordinates (cont)
• In older (30 yrs) literature you will notice lII
and bII listed. This was to distinguish between
new (i.e., correct) and old Galactic
coordinates (before radio astronomy cleared up
the question of where the Galactic center
actually is)
• Epoch does not need to be specified
• Orbit period 250 Myr 5 mas/yr

30
Ecliptic coordinates
• Denoted l, b , defined by plane of the solar
system, logical for orbital dynamics and
satellite data

Dust in the plane of the solar system, which is
bright at 12 mm
IRAS
31
EGRET all-sky map
EGRET (gt100 MeV)
• 1.4 Mg, 60 interstellar emission from the MW
• 10 are cataloged (3EG) point sources

32
Changes in the coordinates!?
• Proper motion record is 10.3/yr
• Precession wobbling of axis due to pull of sun
and moon on a non-spherical earth - 50 per year
(25,000 yr period)
• Nutation - smaller effect due to change in
alignment of Moons orbit 9
• Aberration shift due to finite velocity of light
(20)!
• Diurnal and annual parallax (1 deg for moon)
• Refraction by atmosphere up to 35

33
Thats all folks
34
Astronomical catalogs
• The idea is to label sources so you can refer to
them
• No uniform standards, although standards are
being imposed
• Historically, naming was just sequential, e.g.,
HD12345, W49
• Now the convention is to use the telephone
number, with appropriate level of precision,
along with a designator for the origin catalogs
that undergo revisions also have a version
number the J indicates the epoch hence, 3EG
J18355918
• One exception is transient sources
• E.g., GRBs, for which the name is the date (not
Y2K compliant) of the burst, e.g., GRB030328
• SNR, which are numbered by the year of discovery,
with a letter (or letters) to indicate sequence,
e.g., SNR 1998bw

Henry Draper
Gart Westerhout
35
Units (2) Dates and distances
• JD is Julian Date number of days since noon on
January 1, 4713 BC
• MJD Modified Julian Date JD 2,400,000.5
(i.e., number of days since midnight on November
17, 1858
• Today is MJD 53,314
• (Truncated Julian Date TJD MJD 40,000)
• Distance - Parsec (pc) is the distance at which a
star would have an annual parallax of 1 (3.26
light years)