Sources:1. Adler Planetarium and Astronomy Museum, Chicago2. Hartmut Frommert www.seds.org3. Juergen Giesen www.geoastro.de4. S.W.Digel (SLAC)

Astronomical Observing Techniques:

Coordinate SystemsLecturer: Nigel Douglas

• The Horizon System

• Celestial Sphere:

• Equatorial System

• Distances on the

Celestial Sphere

• Ecliptical Coordinate

system

• Galactic Coordinate

system

• Precession, nutation,

aberration,

refraction, parallax,

etc

The Horizon System(a.k.a. Alt-Az system)• Observer-centered.

• Depends on your

location.

• Measure Azimuth from

N through

East (0-360 deg)

The Horizon System

• Altitude is

measured in decimal

degrees, up from

your horizon

towards your

zenith.

• Also called

Elevation.

The Horizon System

• Your zenith is the

point directly above

your head, at an

altitude of 90º.

• There’s also your

nadir directly below

your feet, at

an altitude of -90º.

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.)

The Horizon System

• Quality of astronomical observations gets poorer as you look

closer to the horizon, because you’re looking through more

atmosphere.

The Horizon System

• When you look straight up, we say that your

observation has an airmass of 1.

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

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.

The Horizon System

• A celestial

object is said to

transit or

culminate when it

crosses your

meridian.

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

Can’t be used to give unique coordinates to astronomical objects - changes with time and with position of observer.

The Horizon System

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.

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.

The Equatorial System

• Project the Earth’s

equator and poles

onto the celestial

sphere.

• A common astronomical

coordinate system for

all observers on

earth!

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 /

’ / ”.

The Equatorial System

• Right Ascension is

measured east along the

celestial equator.

• The reference point for

RA = 0 is the Sun’s

position on the

celestial sphere during

the vernal (spring)

equinox.

Vernal equinox, Mar 21, is the first day of NH spring.

www.crbond.com

The Equatorial System

• Right Ascension is

measured east along the

celestial equator.

• The reference point for

RA = 0 is the Sun’s

position on the

celestial sphere during

the vernal (spring)

equinox.

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

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 aren’t !!!

The Equatorial System• The position of Dubhe (

UMa), the last star in

the bowl of the Big

Dipper, can be given as:

11h 03m 43.5s, 61º 45’ 03

or

11:03:43.5, 61:45:03

or simply

11 03 43.5, 61 45 03.

Why more digits for RA?

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 = (ra cos(decave))2 +

(dec)2

• Here, the units of R.A. and

Dec must be degrees. (Convert

first.)

Distances on the Sky

• For further-separated

objects this equation

doesn’t 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!

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 ra

Equatorial coordinates: summary

• RA, Dec or

• 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]

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

8.5

kpc

Sun

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]

Ecliptic coordinates

• Denoted , defined by plane of the solar system, logical for orbital dynamics and satellite data

IRAS

Dust in the plane of the solar system, which is bright at 12 m

EGRET all-sky map

• ~1.4 M, ~60% interstellar emission from the MW• ~10% are cataloged (3EG) point sources3EG catalog (Hartman et al. 1999)

EGRET(>100 MeV)

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 Moon’s 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’

That’s all folks

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 J1835+5918

• 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

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)