Transcript Document

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)
• 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
z
sec(60º) = 2
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
The Horizon System
Can’t be used to give unique coordinates to astronomical
objects - changes with time and with position of observer.
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 (a 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 = (Dra  cos(decave))2 + (Ddec)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 Dra
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]
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
8.5 kpc
– (0,0) is direction to the Galactic center
– (180,0) is the anticenter
Sun
Powell
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 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
EGRET all-sky map
EGRET
(>100 MeV)
• ~1.4 Mg, ~60% interstellar emission from the MW
3EG catalog (Hartman et al. 1999)
• ~10% are cataloged (3EG) point sources
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
Henry Draper
Gart Westerhout
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;
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)