Transcript Local Horizon View

Finding celestial objects in our night
sky … … requires knowing celestial coordinates,
based on the time of night, and our location
Every star, cluster, nebula, galaxy,
radio source, and quasar has a
position
in the night sky. All the Solar System
objects - the Sun, the Moon, the other
planets, asteroids, and comets have
their
own motion across the background of
stars, so for all these objects their sky
position changes hourly or daily but
can
be mathematically predicted.
All the textbooks, star charts,
planispheres and "GOTO" computers
refer to sky position coordinates :
called Right
Ascension andDeclination.
How can you visualize them on the
night sky?.
Meridian
Your Zenith and Meridian – Looking up
and towards your meridian – The North
South line…
Zenith
Zenith:
Because the sky (celestial sphere) is
constantly in motion, due to the Earth's
rotation, the stars at your zenith are
constantly changing. Regardless, your
zenith
is always overhead - straight up. Your
zenith
is a useful point in the sky because it helps
to define your meridian.
Meridian is the important North/South line
through your zenith and also through
both celestial poles. We look at our celestial
objects while we are oriented along our
North/South meridian
Notice that both your zenith and your
meridian are determined by you and not by
things like Right Ascension or the stars.
Granted, Polaris will always be on your
meridian but that is because it happens to
be
the center of rotation of the celestial sphere.
http://www.synapses.co.uk/astro/earthm
ot.html
Meridian
Our Observing Latitude determines
what celestial objects are seen
above our local horizon
For our location at 45 degrees
latitude, the pole star is at
altitude 45 degrees as
shown to the right. We can
see that when we look up.
This diagram shows that the altitude of Polaris
above the horizon is the same as the
observer's latitude. Note that the lines
drawn to Polaris are parallel because
Polaris is very far away. The direction to
Polaris from the center of Earth is very
nearly the same as from the observer's
position.
Polaris 45
degree up
Local
Horizon
Our Observing Latitude determines
what celestial objects are seen
above our local horizon
Polaris is always above our horizon and since it is at the
pole, it is relatively fixed in the sky during the night.
All stars rotate around this axis.
Using geometry, it is easy to show that the angle
Polaris or the celestial pole makes with the
horizon is equal to the observer's latitude.
In the diagram, the angle is the observer's
latitude. The pole and the equator are at right
angles, or Since the angles in a triangle add to
180°, we know that
d + a = 90
c = b (AIT Alternate Interior Angles of || are equal)
a = 90 –d
a + b + 90 = 180 (sum angles triangle)
(1) a + b = 90
substitute for a in (1):
90 – d + b = 90
d=b
and…
c=d
pole star altitude = latitude.
which means that the angle between the pole and the
horizon (c) is the same as the observer's latitude.
This fact was used by navigators at sea, who could
easily find their latitude by measuring the positions
of the stars.
Objects on your
Meridian
Everything in the sky left of your Meridian is
RISING and everything right of your
Meridian is SETTING, just like the Sun
does.
(In the southern hemisphere, your large area
of sky is facing north, stars rise in the east
(on your right) and set in the west (on your
left).
Everything on your Meridian has therefore
reached its HIGHEST point in the sky
tonight,
and is therefore at its best for viewing since
it
is as far as it can be away from the (murky)
horizons.
Observers in the northern hemisphere orient
their observatories so that the telescope
faces
south because there is a larger surface
area of celestial sphere ( i.e the band of sky
)
from the north pole to the southern horizon
then from the north pole to the
northern horizon. Stars are said to
CULMINATE on your meridian.
http://calgary.rasc.ca/radecl.htm#ra
Side view of Declination lines for an observer at 45° Latitude:
- they are all parallel
- the circles get smaller towards the Celestial Poles
Looking South
Northern observers
face south to observe
star and planets
culminate in the south
along their North/South
meridian
Here the orange line of the
planets (and many comets
and
asteroids) is the ecliptic.
The ecliptic rises
highest overhead in
the
SOUTH for northern
observers.
Star Location: Altitude above Horizon
Star altitude depends on the
Declination or (Dec)
Altitude of Pole Star = Our geographic latitude.
The altitude of any other star
transiting due South on the MERIDIAN
Altitude = Co-latitude + Declination
Celestial Equator
co-latitude
Due South
Local Horizon View:
Altitude of Regulus = 45 + 11deg Declination = 56
deg
Declination ALWAYS measured from
celestial equator to star.
Right Ascension
If Declination is the "up-down" coordinate, then
what is the "left-right" coordinate?
The answer is Right Ascension.
Each of these curved lines is like a N-S
longitude line on the Earth - they all
meet at the poles and they all cross the
Equator at a 90° angle. But how to label
them?
They are not nearly as fixed in the sky
as Declination, since the Earth is
constantly rotating bringing new grid
lines up from the East, and loosing
those low in the West. So a fixed
degree system based on the Pole and
the Equator just won't work. They need
to be tied into the fact that Earth rotates
every 24 hours and this is an amount
of time.
Sidereal Rate and Hour Angle
Each object is catalogued as being
at a certain set of coordinates in
(RA,DEC). For objects visible at
your latitude at a certain time of
year (and night) the object will
appear at a certain "hour angle“
east or west or your meridian for a
given time.
The Right Ascension of the object stays
with the object and comes into view at the
appointed hour!
If you stood outside and looked at
the sky for several hours you would
see the stars seem to move across
your Meridian from East to West at
that rate. This is called Sidereal
Rate, and it is the rate used in
equatorial telescope mounts.
Astronomers used to have to know
their LST (Local Sidereal Time) to
see if it matched up with the Right
Ascension of the object for that time
of year. …
ECU does the Coordinate Transformations
However ECU does the coordinate
transformations from an objects (Right Ascension,
Declination) to your local (Altitude and
Azimuth) For
•
a given latitude,
•
time of year and night
ECU calculates all the positions of
celestial objects that appear above
your horizon
(Alt,Az) = f(RA,Dec,LST,Latitude)
Simple checks for objects near your meridian
Zenith
NP
Celestial
Equator
CoDec
Dec
Lat
To check the altitude
For objects North of the Celestial Pole and
CULMINATING (on the meridian)
Altitude = CoLatitude+ Declination if < 180
…else
Altitude = 180 - (CoLatitude + Declination)
For Circumpolar stars:
Lower Culmination:
Altitude = Latitude – Dec
CoLat
Horizon
To check Right Ascension – with respect to your
Meridian (and Local Sidereal Time)
Hour Angle (where the object is East/West of Meridian) =
RA – LST
If RA = LST, the object is on the meridian
(Off the meridian, you must use
spherical trigonometry)