Celestial Coordinates
To find where an object like a pulsar is in the sky, we need to use a star map. You can
make starmaps for your location and time at Your Sky
(http://www.fourmilab.ch/yoursky/). But to find a pulsar’s position at any chosen time
(not necessarily the current month) we can find out the pulsar's CELESTIAL
COORDINATES
So Celestial Coordinates are to the sky, as geographical coordinates (latitude and longitude) are to
the Earth.
Review of Geographical Coordinates
The Earth's spin determines certain special locations on the Earth. The spin axis goes through the
North and South Poles, and midway between them, the equator circles the Earth.
1. Latitude
To indicate how far north or south we are on Earth,
we specify location in degrees of latitude.
What's the latitude of the North Pole? Of the
equator? If you answered 90 degrees North for the
North Pole and 0 degrees for the equator, that means
you have a basic understanding of latitude. The
latitude lines are all parallel lines running east to
west and they are numbered from 0 to 90 degrees
increasing from the equator to the North Pole (0 to 90 degrees towards the South Pole). You cannot see
latitude lines on the ground--they are just a way of
measuring how far north or south of the equator
something is.
2. Longitude
To indicate how far east or west something is, you
specify longitude in degrees. Longitude lines run
perpendicular to the latitude lines. Unlike latitude,
where there is a unique zero place (the equator)
determined by the spinning of the Earth, there is no
unique longitude determined by the spinning Earth.
All longitude lines, which extend all the way from the
North Pole to the South Pole, are pretty much alike
and they all swing by under the Sun in a 24 hour day.
A zero longitude was arbitrarily chosen to be the
longitude line that runs through Greenwich, England.
Unlike latitude, which runs from 0 to 90 degrees north
and 0 to 90 degrees south, longitude lines can be
numbered in a complete circle of 360 degrees, but we
usually extend them only 180 degrees east of Greenwich and 180 degrees west of Greenwich.
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3. Minutes and seconds of arc
Aren't minutes and seconds just for telling time? NO! For giving locations more precisely, degrees of
latitude or longitude are subdivided into minutes--each degree has 60 minutes. They are often called
minutes of arc, because they measure an angle, just like degrees. Each minute is further divided into
60 seconds of arc.
So anywhere on Earth can be located by specifying its geographic coordinates in latitude and
longitude. For example, San Francisco, California is 122 degrees, 26 minutes west of Greenwich and
37 degrees 46 minutes north of the equator. This is abbreviated
122 deg. 26' W
37 deg. 46' N
The single quote (') stands for minutes of arc, either latitude or longitude; A double quote mark (")
stands for seconds of arc.
Questions:
1. What are the units of measure for lines of latitude? Describe the possible range of values for
these units. Identify the starting point of this range (the zero mark).
2. What are the units of measure for the lines of longitude? Describe the possible range of values
for these units. Identify the starting point of this range (the zero mark).
3. Are the lines of latitude and longitude fixed or do they change as the earth rotates on its axis?
4. Each degree of latitude and longitude are subdivided into smaller units. Explain this
subdivision.
5. Calculate the total number of minutes of arc in the complete range for lines of longitude.
6. Calculate the total number of seconds of arc in the complete range for lines of longitude.
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Celestial Coordinates
4. Declination
Imagine extending the Earth's axis
infinitely into space, north and south.
Long ago, people believed that there
was a giant sphere to which the stars
were attached--the celestial sphere.
The Earth's axis would pierce that
sphere in two places: the celestial
north pole and the celestial south
pole. If you imagine the surface of the
Earth being inflated like a giant
balloon until it touched the celestial
sphere, the Earth's equator would be
touching the celestial equator.
Even though we know there is no
physical sphere out there holding up
the stars, it's still convenient to think
of an imaginary celestial sphere for
specifying locations of things in the
sky. The "celestial latitude lines"
represent celestial coordinate known
as declination. Just as latitude on Earth increases from 0 to 90 degrees as you move from equator to
Pole, declination in the sky increases from 0 to 90 degrees as you look progressively north from the
celestial equator, and from 0 to -90 degrees as you look progressively south from the celestial equator.
5. Right Ascension
Yup, you guessed it. There is something in the sky corresponding to longitude on Earth. The
corresponding coordinate in the sky is called right ascension. Unlike longitude, which is measured in
degrees and minutes, right ascension is measured in hours and minutes. There are 24 hours of right
ascension corresponding to the full sweep of 360 degrees around the celestial equator. Simple division
will tell you that each hour of right ascension must be equivalent to 15 degrees of arc.
6. Distance
Celestial coordinates specify which direction something is with respect to Earth, but not the distance to
the object. For objects within the solar system (comets, planets, moons, asteroids), distance is often
specified in Astronomical Units (AU)--an AU is the average distance from the Earth to the Sun =
149,597,870 kilometers (92,955,730 miles). For objects outside the solar system, the light year (ly) is
often used. This is the distance light travels in a year; one light year is equal to 63,240 AU.
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Questions:
1. What is the celestial sphere?
2. The north geographical pole extended into the sky is called
3. The south geographical pole extended into the sky is called
4. Describe the celestial equator.
5. What is the latitude equivalent for the celestial sphere? Is the range for this the same as it is on
earth? What is the range?
6. What is the longitude equivalent for the celestial sphere? Is the range for this the same as it is
on earth?
7. What units are used to measure lines of right ascension?
8. How many total hours are there in the range of RA?
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Wikipedia
Right ascension (abbrev. RA; symbol α) is the astronomical term for one of the two coordinates of a
point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the
declination.
Explanation
Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude
measure an angle from a zero point on an equator. For right ascension, the zero point is known as the
first point of Aries, which is the place in the sky where the Sun crosses the celestial equator at the
March equinox. Right ascension is measured continuously in a full circle towards the east.
Any units of angular measure can be used for right ascension, but it is customarily measured in hours,
minutes, and seconds, with 24 hours being equivalent to a full circle. The reason for this choice is that
the Earth rotates at an approximately constant rate. Since a complete circle has 360 degrees, an hour of
right ascension is equal to 1⁄24 of this, or 15 degrees of arc, a single minute of right ascension equal to
15 minutes of arc, and a second of right ascension equal to 15 seconds of arc.
Right ascension can be used to determine a star's location and to determine how long it will take for a
star to reach a certain point in the sky. For example, if a star with RA = 01:30:00 is at a location's
meridian, then a star with RA = 20:00:00 will be in the meridian 18.5 hours later.
For observation purposes, the apparent right ascension is generally used. This gives the position of the
body corrected for the delay from the time of light to travel from the body being observed to the Earth.
The tilt of the Earth's axis gradually rotates over time. This effect, known as precession, causes the
measured right ascension and declination of even a perfectly stationary celestial object to change
noticeably over the span of decades. Therefore, equatorial coordinates are inherently relative to the
year of their observation. Astronomers always specify equatorial coordinates with reference to a
particular epoch. An astronomer comparing coordinates from different epochs must mathematically
rotate one of the coordinate systems to match the other, or rotate both to match a common epoch.
The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J"
indicates that it is a Julian Epoch. Prior to J2000.0, astronomers used the successive Besselian
Epochs B1875.0, B1900.0, and B1950.0.
Questions:
1. What is the starting point for declination? (the zero mark)
2. What is the starting point for right ascension? (the zero mark)
3. For the pulsar J0820-1350 list
RA
Dec
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4. Why does the starting point for right ascension change over time?
5. Pulsars orbit the center of their respective galaxies just like other stars. So they are moving.
Some are traveling faster than others. The location of a pulsar in the sky does not change much
(but some) on a yearly basis due to its velocity since it is so far away. If we are to determine
its location in the sky using the RA and Dec coordinate system the earth’s axis would have to
be stationary. It is not. The earth processes on its axis in a regular pattern with a period of
about 26,000 years. That’s a long time, but this small change in the orientation of the earth
changes the location of the pulsar by a little (but measurable) bit. How do astronomers name
pulsars to indicate the time in which the pulsars coordinates are matched to a particular RA and
Dec?
Use the Celestial Sphere to locate the pulsar based on its name. Complete the following table to
identify the constellation in which the pulsar is located.
Pulsar Name
Constellation
Vela Pulsar
J0835-4510
CRAB Pulsar
B0531+21
PSC Student: Hannah Mabry
J1820+0159
PSC Student: David Dunkum
J2136-1612
E@H
J1907+0510
E@H
J1910+1017
E@H
J1950+2416
E@H
J1952+2512
5 point BONUS: What is E@H?
hint: http://albert.phys.uwm.edu/radiopulsar/html/BRP4_discoveries/
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