Watch, When, and Where

1
Watch, When, and Where

• Or Universal Time, Julian Date, and Celestial
  Coordinates.

2
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

3
WatchUNIVERSAL TIME
4
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.

5
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.

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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)
7
When JULIAN DATE
8
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.

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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. y1984, m14, d17.25, A19, B-13,
C 724656, D 459 JD 2446113.75
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WhereASTRONOMICAL COORDINATES
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Types of Coordinates

• Topocentric System (Altitude and Azimuth)
• Equatorial System (Right Ascension and
  Declination)
• Galactic System (Galactic Latitude and Longitude)

12
Viewing the Sky
13
Actual Distance and View
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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.

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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.

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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.

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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.

18
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.

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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.

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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 1015 PM
on July 5, 2001. What would be its azimuth and
altitude?

M-4
SW
SE
S
Az 180 Alt 30
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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-4s Right
Ascension and Declination?
M-4
16 h 30 m -23
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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-4s Galactic
longitude and latitude?
M-4
350 20
23
Referencing - Galactic View

M-4
This view shows only the galactic system. Notice
how the center of the galaxy runs along the
galactic equator line.
24
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.

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Referencing - Equatorial Sky
This view shows the the galaxy projected onto the
equatorial system. The celestial poles are at
the top and bottom.

26
Referencing - Horizon Sky
This view shows the the galaxy projected onto the
horizon system.

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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
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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-1cos 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.
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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-1cos 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
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Additional Conversions
Horizon
Universal Time
Equatorial
Galactic
Ecliptic
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Good Source and More..
Practical Astronomy With Your Calculator By Peter
Duffett-Smith