Download ASTR 111 Lab Manual - Ohio Wesleyan University

Using a Celestial Globe
Robert Harmon
Department of Physics and Astronomy
Ohio Wesleyan University
Because the Sun, Moon, planets, stars and other celestial objects are so far away from us,
they all appear to lie at the same distance from our eyes. For this reason it is very
convenient and useful to imagine that all celestial objects lie on the inside surface of an
enormous sphere centered on the Earth. We call this imaginary sphere the celestial
sphere. A celestial globe is a representation of the celestial sphere, a map of the sky in the
same way that an Earth globe is a map of the Earth.
An important difference between a celestial globe and an Earth globe is the fact that we
view the sky from the inside, since Earth lies inside the celestial sphere. Constellations
are printed backwards on the surface of the celestial globe because we view the globe
from the outside. Of course, it isn’t really possible to view the sky from outside it. Keep
in mind as you use your globe that the observer is always inside the globe at its center,
never on its surface. After all, you can’t stand on the sky!
Examine your globe. Note that it is pivoted at the north and south celestial poles. These
are the projections of the north and south poles of the Earth onto the celestial sphere.
Halfway between the celestial poles lies the celestial equator, which is the projection of
the Earth’s equator onto the celestial sphere. The Earth spins from west to east on an axis
which passes though the north and south poles. However, because Earth takes a whole
day to spin once around, we do not feel this motion. Instead, the celestial sphere appears
to turn around us in the opposite direction, from east to west, around an axis passing
through the north and south celestial poles. This is why the Sun, Moon, planets and stars
appear to rise and the east and set in the west (except for circumpolar stars, which you
will learn about below).
The horizontal metal ring surrounding the globe represents the astronomical horizon,
which divides the sky into the half which the observer can see and the half which cannot
be seen. The astronomical horizon is the horizon you would see if there were no trees,
buildings, mountains, and so forth, so that you had a perfectly flat horizon.
Because the Earth orbits the Sun, we see the Sun in front of different stars at different
times of the year. As a result, the Sun traces out a circular path on the celestial sphere
called the ecliptic. The Earth orbits the Sun in a plane, and the ecliptic is the projection of
this plane onto the celestial sphere. The band about 8 degrees wide on either side of the
ecliptic is called the zodiac. The constellations through which the ecliptic passes are
called zodiacal constellations.
The ecliptic on your globe is marked off with dates; these are the dates on which the Sun
lies at the corresponding locations. Locate the ecliptic on your globe. Note that the
ecliptic is tilted by about 23½º with respect to the celestial equator. This arises because
the Earth’s own equator is tilted by 23½º with respect to the plane of Earth’s orbit around
the Sun. The two points at which the ecliptic intersects the celestial equator are called the
1
equinoxes. The vernal equinox is the point at which the Sun crosses the celestial equator
heading north, while at the autumnal equinox the Sun crosses the celestial equator
heading south. Locate the two equinoxes on your globe. When the Sun is at the vernal
equinox, spring officially begins in the Northern Hemisphere, while autumn in the
Northern Hemisphere begins when the Sun is at the autumnal equinox. (In the Southern
Hemisphere the seasons are reversed. Since the ancient astronomers who names the
equinoxes lived north of the equator, the names reflect a “Northern Hemisphere bias.”)
On Earth we can give the location of a point on its surface by specifying its latitude and
longitude. Latitude specifies north-south location, while longitude specifies east-west
location.
The latitude of a point is the angle as seen at the center of the Earth between that point
and the equator. Note that we must specify whether the point lies north or south of the
equator, which we do by appending “N” or “S” to the angle. The north pole has latitude
90º N, while the south pole has latitude 90º S. Of course, the equator itself has latitude 0º.
The meridian through a point, which we need in order to define its longitude, is the circle
on the Earth which passes through that point and also through the north and south poles.
An international conference in 1884 chose the meridian through the Royal Observatory in
Greenwich, England as the Prime Meridian, i.e., the meridian having a longitude of 0º.
The longitude of any other location is the angle between the meridian through that
location and the Prime Meridian. Longitudes are measured east and west of Greenwich.
For example, Delaware is at latitude 40º 17 N, longitude 83º 04 W. (1 minute of arc,
written 1, is 1/60 of a degree. Thus 40º 17means 40 1 76 0 .)
In a similar way, astronomers need to be able to precisely specify the locations of
celestial objects on the celestial sphere, so that other astronomers will be able to find
them. To do so, we use declination and right ascension.
The declination of a star (or other celestial object) is the angle between the star and the
celestial equator, and is analogous to latitude on the surface of the Earth. Instead of
appending “N” or “S” to the declination, we use the convention that objects north of the
celestial equator have positive declination, while objects south of the celestial equator
have negative declination. On your globe, declination is indicated on the verticallyoriented metal ring surrounding the sphere. Be careful: on the other side of the
declination scale, this ring indicates the codeclination, which specifies the angle from the
nearest celestial pole instead of from the celestial equator. Thus, the co-declination is 0º
at the celestial poles and 90º at the celestial equator. Be sure that you use the declination
scale, which is the one that reads 0º at the celestial equator and 90º at the celestial poles,
when you are determining declinations! Declination is also indicated directly on the globe
itself in the form of declination circles parallel to the celestial equator at 10º intervals.
Right ascension on the celestial sphere is analogous to longitude on the surface of the
Earth. Just as we use Greenwich as the zero point for longitude on Earth, we need a zero
point for right ascension. This point has been arbitrarily chosen to be the vernal equinox.
2
An important difference between longitude and right ascension is that longitude is
measured in degrees, while right ascension is measured in units of time. To understand
this, we must digress a little.
For an observer, the meridian is the circle on the sky which stretches from due north to
due south and passes through the zenith, which is the point directly overhead. Note that
this is not the same thing as a meridian of longitude on the surface of Earth. The vertical
brass ring on which declinations and codeclinations are marked represents the meridian
on your celestial globe.
The sidereal day is the time required for the vernal equinox to go from the meridian back
to the meridian again due to the Earth spinning on its axis. Our clocks and watches, on
the other hand, are based on the mean solar day, which is the average time it takes the
Sun to go from the meridian back to the meridian again. The solar day is about 4 minutes
longer than the sidereal day because the Sun moves from west to east along the ecliptic
from day to day due to Earth’s revolution around the Sun, which is opposite to the eastto-west motion of the Sun due to Earth’s rotation on its axis. As a result, the Earth has to
turn through a little more than 360º in order to bring the Sun back to the meridian again,
while it only has to turn 360º to put the vernal equinox back on the meridian.*
An hour of right ascension is the angle through which the celestial sphere turns in an hour
of sidereal time. Similarly, a minute of right ascension is the angle through which the
celestial sphere turns in one minute of sidereal time, while a second of right ascension is
the angle through which the celestial sphere turns in one second of sidereal time.
Another difference between right ascension and longitude is that we do not explicitly
distinguish between east and west right ascensions. Instead, we measure all right
ascensions in the eastward direction away from the vernal equinox. A star with a right
ascension of 23h is 23 hours east of the vernal equinox, which is the same as 1 hour west
of the vernal equinox.
Right ascension is indicated on your globe along the celestial equator. Hour circles pass
perpendicular to the equator, meeting at the poles. In between the hour circles the equator
is marked off in 10-minute intervals.
*
Actually, the Earth turns through slightly less than 360º in one sidereal day, because the
vernal equinox moves very slowly along the ecliptic from east to west. This is because
the Earth’s axis of rotation wobbles like a top in a motion called precession, taking
26,000 years to complete one cycle. You might have heard that a year is the time it takes
the Earth to go once around the Sun. However, our calendars are actually based on the
tropical year, which is the time between successive vernal equinoxes. Due to precession,
this is less than the time it takes Earth to go once around the Sun by 1 part in 26,000. This
difference is small but significant. If we did not base the year on the vernal equinox, the
dates of the equinoxes would slowly change over the centuries, and thus the seasons
would start and end at different times of the year!
3