Watch, When, and Where: Watch, When, and Where Or Universal Time, Julian Date, and Celestial Coordinates.
Overview: Overview What are the things you need to know in order to give a location of the origin of a cosmic ray?
Universal Time
Julian Date
Local Coordinates
Watch:UNIVERSAL TIME: Watch: UNIVERSAL TIME
Universal Time: Universal Time The times of various events, particularly astronomical and weather phenomena, are often given in "Universal Time" (abbreviated UT) which is sometimes referred to, now colloquially, as "Greenwich Mean Time" (abbreviated GMT).
When a precision of one second or better is needed, however, it is necessary to be more specific about the exact meaning of UT. For that purpose different designations of Universal Time have been adopted. In astronomical and navigational usage, UT often refers to a specific time called UT1, which is a measure of the rotation angle of the Earth as observed astronomically.
However, in the most common civil usage, UT refers to a time scale called "Coordinated Universal Time" (abbreviated UTC), which is the basis for the worldwide system of civil time. This time scale is kept by time laboratories around the world, including the U.S. Naval Observatory, and is determined using highly precise atomic clocks.
UTC and Local Time: UTC and Local Time UTC is the time distributed by standard radio stations that broadcast time, such as WWV and WWVH. It can also be obtained readily from the Global Positioning System (GPS) satellites.
Countries lying on meridians east or west of the Greenwich meridian do not use GMT as their local civil time. It would obviously be impractical to do so as the local noon, the time at which the sun reaches its maximum altitude, gets earlier or later with respect to the local noon on the Greenwich meridian.
To avoid confusion, the world is divided into time zones, each zone corresponding to a whole number of hours before or after UT. This time is known as your local time.
Conversion to UTC: Conversion to UTC It is often convenient in making astronomical calculations to use UT and local time may be converted into UT in the following manner. Local Time = Universal Time - 6 hr (central standard time)
- 5 hr (central daylight time)
Local Time = Universal Time - 7 hr (mountain standard time)
- 6 hr (mountain daylight time)
When: JULIAN DATE: When: JULIAN DATE
The Julian timeline: The Julian timeline Julian dates (abbreviated JD) are simply a continuous count of days and fractions since noon Universal Time on January 1, 4713 BCE (on the Julian calendar). Almost 2.5 million days have transpired since this date. Julian dates are widely used as time variables within astronomical software.
Calendar dates — year, month, and day — are more problematic. Various calendar systems have been in use at different times and places around the world. Primarily there are two calendars: the Gregorian calendar, now used universally for civil purposes, and the Julian calendar, its predecessor in the western world.
Sometimes the modified Julian date, MJD, is quoted. This is equal to the JD - 2 400 00.5; MJD zero therefore began at 0h on November 17th 1858.
Conversion to Julian Days: Conversion to Julian Days The Julian date of any day in the Julian calendar may be found by the method below. Set y= year, m = month and d = day
If m = 1 or 2 subtract 1 from y and add 12 to m
Calculate A= integer part of y/100; B = 2 - A+ integer part of (A/4)
Calculate C = integer part of 365.25 • y
Calculate D = integer part of 30.6001 x (m + 1)
Find JD = B + C + D + d +1720994.5 Example: Calculate the Julian date for February 17.25, 1985.
y=1984, m=14, d=17.25, A=19, B=-13, C= 724656, D = 459
JD = 2446113.75
Where:ASTRONOMICAL COORDINATES: Where: ASTRONOMICAL COORDINATES
Types of Coordinates: Types of Coordinates Topocentric System (Altitude and Azimuth)
Equatorial System (Right Ascension and Declination)
Galactic System (Galactic Latitude and Longitude)
Viewing the Sky: Viewing the Sky
Actual Distance and View: Actual Distance and View
Topocentric (Horizon) System: Topocentric (Horizon) System Azimuth (A)
This is the direction of a celestial object, measured clockwise around the observer's horizon from north. So an object due north has an azimuth of 0°, one due east 90°, south 180° and west 270°. Azimuth and altitude are usually used together to give the direction of an object in the topocentric coordinate system.
Topocentric System: Topocentric System Altitude (h)
The angle of a celestial object measured upwards from the observer's horizon. Thus, an object on the horizon has an altitude of 0° and one directly overhead has an altitude of 90°. Negative values for the altitude mean that the object is below the horizon.
Equatorial (Celestial) Coordinates: Equatorial (Celestial) Coordinates The term "Celestial Sphere" describes the appearance of the nighttime sky. The stars revolve in diurnal rotation without changing their positions relative to one another. Because our eyes are not sensitive to the varying "distance of the stars" away from us, the stars appear to lie all at the same large distance away. This leads to the concept of the sky and stars as a sphere concentric with the earth and rotating around it. This apparent rotating sphere carrying the stars on its surface is known as the Celestial Sphere.
Equatorial Coordinates: Equatorial Coordinates Right Ascension (a)
In the sky with right ascension (celestial longitude) we need some point to play a similar role to that of Greenwich, England for terrestrial longitude. Astronomers have chosen the Vernal Equinox to define the starting point for the measurement of right ascension. The Vernal Equinox is the point where the sun appears to cross the Celestial Equator at the beginning of spring.
Equatorial Coordinates: Equatorial Coordinates Declination (d)
Declination works on the surface of the Celestial Sphere much like latitude does on the surface of the earth, that is, declination measures the angular distance of a celestial object north or south of the Celestial Equator. Lines of declination are analogous to parallels of latitude on the earth. An object lying on the Celestial Equator has a declination of 0°. The declination increases as you move away from the Celestial Equator to the Celestial Poles. At the North Celestial Pole therefore, the declination is + 90°.
Galactic Coordinates: Galactic Coordinates Here the fundamental line is the galactic circle, a great circle represented by the course of the “Milky Way”. The galactic circle or equator, intersects the celestial equator at an angle of about 63 degrees. Galactic longitude (l), is measured from the center of the galaxy, eastward along the galactic equator. Galactic latitude (b), is measure on the great circle through the object and the galactic pole, and counted north (+) or south (-) with respect to the galactic equator.
Referencing - Horizon View: Referencing - Horizon View This is an image of the night sky looking directly south from Lincoln. In the middle of the image is the object M-4, a globular cluster in Scorpius. This is how the observer would see the cluster relative to the horizon at 10:15 PM on July 5, 2001.
What would be its azimuth and altitude? M-4 S SW SE Az 180 ° Alt 30°
Referencing - Horizon and Equatorial View: Referencing - Horizon and Equatorial View This view shows the same image as before but with the addition of the equatorial grid overlayed on the horizon view.
What is M-4’s Right Ascension and Declination? M-4 16 h 30 m
-23°
Referencing - Equatorial and Galactic View: Referencing - Equatorial and Galactic View This view shows the horizon removed and the equatorial aligned with the celestial equator. The galactic system has been inserted over the equatorial system.
What is M-4’s Galactic longitude and latitude? M-4 350°
+20°
Referencing - Galactic View: Referencing - Galactic View This view shows only the galactic system. Notice how the center of the galaxy runs along the galactic equator line. M-4
Referencing - Galactic Sky: Referencing - Galactic Sky This view shows the galactic system projected for the entire sky. The galactic poles are at the top and bottom of the image.
Referencing - Equatorial Sky: Referencing - Equatorial Sky This view shows the the galaxy projected onto the equatorial system. The celestial poles are at the top and bottom.
Referencing - Horizon Sky: Referencing - Horizon Sky This view shows the the galaxy projected onto the horizon system.
Equatorial to Galactic Coordinate Conversion: Equatorial to Galactic Coordinate Conversion In previous pictures you should have noticed that all of the coordinate systems are based on spheres. Conversion from one system to another is based on the use of spherical trigonometry. The set of equations below forms the basis of spherical trigonometry, which governs, amongst other things, the change in coordinates consequent upon a rotation of the axes. The equations are useful when the angles are less than 180º, and the sides of the figures are formed from arcs of great circles. cos a = cos b cos c + sin b sin c cos A
sin a /sin A = sin b / sin B = sin c / sin C
sin a cos B = cos b sin c - sin b cos c cos A
Equatorial to Galactic Coordinate Conversion: Equatorial to Galactic Coordinate Conversion In order to list a cosmic rays origin in galactic coordinate you must know its equatorial coordinates. The conversion formulas are:
b = sin-1{cos d cos (27.4) cos (a - 192.25) + sin d sin (27.4)}
l = tan-1{(sin d - sin b sin (27.4))/ cos d sin (a - 192.25) cos (27.4)} + 33
The numbers come from the following facts about our Galaxy: north galactic pole coordinates a = 192º 15’, d = 27º 24’; ascending node of galactic plane on celestial equator l = 33º. These are 1950.0 coordinates.
Equatorial to Galactic Coordinate Conversion: Equatorial to Galactic Coordinate Conversion Example: What are the galactic coordinates of a star whose right ascension and declination are a = 10 h 21m 00s and d = 10º 03’ 11”?
b = sin-1{cos d cos (27.4) cos (a - 192.25) + sin d sin (27.4)}
l = tan-1{(sin d - sin b sin (27.4))/ cos d sin (a - 192.25) cos (27.4)} + 33
b = 51.122268º or 51º 07’ 20”
l = 232.247874 or 232º 14’ 53”
Additional Conversions: Additional Conversions Horizon Equatorial Galactic Ecliptic Universal Time
Good Source and More…..: Good Source and More….. Practical Astronomy
With Your Calculator
By
Peter Duffett-Smith