Download - Finding celestial objects in our night sky: Right Ascension Declination Local Time of Day
Finding celestial objects in our night sky:Right AscensionDeclinationLocal Time of Day
Every star, cluster, nebula, galaxy,radio source, and quasar has a
positionin the night sky. All the Solar Systemobjects - the Sun, the Moon, the
other planets, asteroids, and comets have
theirown motion across the background
of stars, so for all these objects their
sky position changes hourly or daily but
canbe mathematically predicted.
All the textbooks, star charts,planispheres and "GOTO" computersrefer to sky position coordinates :called Right Ascension and
Declination.
How can you visualize them on thecelestial sphere?
Meridian
Your Zenith and Meridian from the Horizon
The North South line…
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 linethrough your zenith and also throughboth celestial poles. We look at our celestial objects while we are oriented along our North/South meridian
Notice that both your zenith and meridianare determined by you on your Horizon andnot by absolute Right Ascension, Declination
Granted, Polaris will always be on yourmeridian but that is because it happens to be the center of rotation of the celestial sphere.
Celestial Coordinates Right Ascension and Declination
• We map celestial coordinates with the aid of the concept of a celestial sphere. This is an imaginary ball larger than the entire visible universe. Imaginary lines are drawn from the Earth through celestial objects, extending beyond them until the lines touch the surface of the celestial sphere. These points mark the apparent positions of those objects given in star charts, catalogs and almanacs.
• The position of a celestial object is given by its Right Ascension (RA) and Declination (Dec) in the same way as our position on earth is given by our Longitude and Latitude.
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 Polarisabove the horizon is the same as the observer'slatitude. Note that the lines drawn to Polaris areparallel because Polaris is very far away. The direction to Polaris from the center of Earth isnearly 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
c tothe Celestial Pole (Polaris) makes with the horizon
isequal to d, the observer's latitude.
In the diagram, angle d is observer's latitude. The pole and the equator are at right angles. Altitude Polaris = Latitude of ObserverProof : Angle c = Angle d (Latitude)
d + a = 90c = b (AIT Alternate Interior Angles of || are
equal)a = 90 –da + b + 90 = 180 (sum angles triangle)(1) a + b = 90substitute for a in (1): 90 – d + b = 90 d = band…c =b and d = bTherefore c = dpole star altitude = latitude.
This fact was used by navigators at sea, who could
easily find their latitude by measuring the positions
of the stars.
Astronomical Navigation (Latitude)
When a star culminates on the navigator’s meridian,the observed altitude plus the of declination the starat the time of meridional crossing gives thenavigator’s latitude according to:
Latitude = 90 – Altitude + Declination
Latitude (but not Longitude) could be found to a fair precision (about 30 miles) byobservation of the meridian altitudes of theSun and certain stars, such as the pole Starabove the horizon.
Courtesy Man Is Not Lost , D.H. Sadler Her Majesty’s Stationary Office 1968
Stars Culminate on your Meridian
Everything in the sky left of your Meridian isRISING and everything right of yourMeridian 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 yourleft).
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.
When the Star crosses the Meridian, it is the single
point of highest altitude.Stars are said to CULMINATE on your meridian If the star is off the meridian, there are 2
altitudes for it:• east of the meridian • west of the meridian.
Observers in the northern hemisphere orienttheir observatories so that the telescope faces
South
(courtesy http://calgary.rasc.ca/radecl.html )
Side view of Declination lines for an observer at 45° Latitude:135 degrees of sky from the north pole to the southern horizonOnly 45 degrees of sky from north pole to the northern horizon
Star Location: Altitude above Horizon
Star altitude depends on theDeclination 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
Local Horizon View:Altitude of Regulus = 45 + 11deg Declination = 56 deg
Declination ALWAYS measured from celestial equator to star.
Due Southco-latitude
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 (Earth Centered Universe Computer Program) does the Coordinate Transformations
However ECU does the coordinatetransformations 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 aboveyour horizon
(Alt,Az) = f(RA,Dec,LST,Latitude)
Simple checks for objects near your meridian
NP
Celestial Equator
Zenith
Horizon
To check the altitude For objects North of the Celestial Pole and CULMINATING (on the meridian)Altitude = CoLatitude+ Declination if < 180…elseAltitude = 180 - (CoLatitude + Declination)
For Circumpolar stars:Lower Culmination:Altitude = Latitude – Dec
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
CoLat
Dec
Lat
CoDec
(Off the meridian, you must use spherical trigonometry)