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```									Chapter 13

Constellation Charts

Your TA will distribute two constellation charts; they are named SC001 and SC002.
SC001 shows the equatorial region of the sky (+60◦ to −60◦ declination). SC002
shows the north circumpolar region. These charts give the positions of the brighter
objects in the sky, and also include the path of the Sun throughout the year. Like
the Distant Suns planetarium program and your planisphere, these charts represent
another way to map the sky.

13.1     Chart SC001

First look at SC001. The hatched line running horizontally across the chart denotes
the celestial equator. The celestial equator is the projection of the Earth’s equator
onto the celestial sphere. The coordinates along this axis are in right ascension
(R.A.) and are denoted in hours instead of degrees. Since the Earth rotates once
every twenty-four hours, the relative positions of the stars can be conveniently mea-
sured in these coordinates and their positions easily predicted as a function of time.
Note that east is to the left and west to the right on the chart. This is because it is
meant to be held over your head, facing southwards. Also note that R.A. increases
from right to left, not left to right. Since the stars move from East to West, if a star
is directly overhead now, then the stars one hour to the West were overhead one
hour earlier. Therefore, earlier times are found to the right in the westerly direction.
Take a few moments and study the map and convince yourself that this all makes
sense. It can seem very confusing at ﬁrst glance.
Along the side of the map are numbers running from 0◦ at the equator to +60◦ to
the North and −60◦ to the South. These coordinates are measured in degrees of
declination and are analogous to latitude on Earth. They stop at 60◦ , rather than
going to 90◦ to avoid the distortions that would be introduced near the poles. If you
use these charts to look at the sky, you must expect some distortion in the shapes
of the constellations.
Find the star at R.A. = 18h 40m and decl. = 39◦ . If it is not Vega, you are not

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reading the chart correctly. (Note: Named stars have either a proper name (e.g.,
Sirius) or are identiﬁed by a Greek letter and the name of the constellation).

13.2       The Ecliptic

The curved hatched line running horizontally across the map is the ecliptic. This is
the apparent path taken by the Sun, through the stars, over the course of a year.
The ecliptic crosses the celestial equator on the date of the vernal and autumnal
equinoxes, which are the ﬁrst day of Spring (∼March 20th) and the ﬁrst day of
Fall (∼September 20th) respectively. The zero coordinate of R.A. has been deﬁned
with respect to the vernal equinox. The dates running along the ecliptic denote the
position of the Sun with respect to the stars throughout the year. Find the ﬁrst
day of spring on the map and verify that this is the case. The Signs of the Zodiac
follow the ecliptic. Examine the ecliptic and ﬁnd all twelve constellations of the
Zodiac.
As you may have noticed, the Zodiac Signs are not well-deﬁned in size and do not
correspond to the traditional dates. They were originally set up such that the Sun
was entering Aries on the vernal equinox1 . Due to the precession of the Earth’s
axis, the position of the vernal equinox has changed over the last several thousand
years.

13.3       Predicting when a Star will be on the Meridian

The meridian is an imaginary line running directly North-South, and always passes
through the North Star Polaris and the zenith point. A star is said to be on the
meridian when it is on this line. The dates along the bottom of the map indicate
which line of RA will be on the meridian at 8 pm on that date.

13.4       Discussion and Analysis

the charts!
1. Give the “proper” names of the following stars along with their approximate
right ascensions and declinations.
κ Ori                    α Boo                      β UMi
2. A line drawn between two stars in the “Big Dipper” (part of Ursa Major)
points toward Polaris. Identify these two stars. Give two names for each.
3. Determine which of the following stars is the brightest.
1
Zero hours Right Ascension is deﬁned as the point where the Sun crosses the celestial equator
at the vernal equinox.
Constellation Charts                                                            137

ǫ Del                 γ Mon                 η CMa
4. List four double stars that are 2nd magnitude or brighter. Also list four star
clusters, four nebulae, and two galaxies.
5. Find the position of the Sun on your birthday. What is the nearest Zodiac
sign? Are you surprised?
6. What is the R.A. and Dec. of the Sun today? What Zodiac sign is it in?
7. What line of R.A. will be on your meridian at 8:00 pm on February 20? Find a
star whose declination is approximately the same as Williamsburg’s latitude.
What will its altitude be? What line of R.A. will be on your meridian at 2:00
am on February 21?
8. If Vega crosses the meridian at midnight, at what time will Deneb cross it?