Download Using Star Charts

In Class Assignments.
Assignment #1 - Reading Star Charts
Getting to know the Heavens, and the motions of Sun, Moon and Stars
If the sky is imagined to be a celestial sphere, then the stars may be represented on a map
in much the same way that the spherical Earth can be mapped onto a flat piece of paper.
The lines of latitude and longitude on a map of the Earth have their equivalent on a map
of the celestial sphere, with the revised names of Declination and Right Ascension. The
same problem of trying to represent a sphere on a flat piece of paper is encountered in
both cases with a Mercator projection, which concentrates on the equatorial region, so the
north and south parts of the map are very distorted. On the other hand a polar projection,
which concentrates on the area around the north or south pole, is limited to that small
region.
There are many different types of star charts available, in a variety of forms, each
designed to get around the problem that the appearance of the sky changes with time. To
use any of them usually requires a little practice. The star chart that you will be
introduced to in this assignment are those published by Sky Publishing Corporation, they
can be purchased in the bookstore. Keep them throughout the year as a valuable and
required reference. Detailed descriptions of them will be given in lectures. It is hoped
that by the end of the assignment you will have enough information to use the charts to
find your way around the night sky. Remember, however, that there is no substitute for
experience. To become familiar with the sky requires you to stand outside a few times,
become adapted to the dark and identify the bright stars. The constellations will soon
become familiar to you, as will the planets that wander through them.
Part A: The Equatorial Chart
One of the star charts in this exercise (the large one) displays the stars found in a broad
band around the celestial equator between declinations 60oN and 60oS. Marked across the
bottom (horizontal) axis is Right Ascension, RA. Moving east (to the left) on the chart
represents increasing hours of right ascension. A vertical line drawn through an hour of
RA represents the local meridian at 8:00pm local standard time on the date that is
printed below the RA hours. The chart then shows the sky on either side of the local
meridian at 8:00pm for the date given. Also drawn on the chart is the ecliptic, a curving
line that represents the approximate path of the sun, moon and planets across the sky.
Note that the dates on the ecliptic are different from those at the base of the chart. The
ecliptic dates indicate where the sun is at any time during the year with respect to the
stars.
To translate the chart into the appearance of the night sky bend the chart into the shape of
a cylinder, printed side in, with the lines marking 12h right ascension at each end
coincident. For an observer at a latitude of 44oN, (e.g. in Toronto), tip the cylinder toward
the north to an angle of 44o. Imagine yourself at the centre of the cylinder looking up at it.
A horizontal plane through your position marks the horizon where it cuts the chart.
During the night the chart rotates around its axis, causing some stars to rise and some to
set. If the positions of the planets are known for the particular date they may be plotted on
the chart as well.
There are some important differences between maps of the Earth and maps of the
celestial sphere, which must be understood in order to use the star charts:

The celestial sphere is always viewed from the inside, so a star chart represents what
you can see, not where you are or where you can go. Remember, you are not on the
star map and are always at the centre of the celestial sphere.

The celestial sphere is seen from the inside out, not the outside in, so maps appear to
be `turned around' compared to maps of Earth. i.e. for the Mercator projection they
show east, rather than west, on the left side.

The entire celestial sphere can be represented on a map, but you can only see the half
of the celestial sphere that is above the horizon at any given time. The half that you
see is different at different times of the night and on different nights through the year,
because of the Earth's rotation and its orbital motion around the sun. To use a star
chart you have to consider the date and the time of night. This is the main reason that
star charts are more difficult to use than maps of the Earth.

If we were at the North Pole, or on the Equator, it would be simple to describe what
part of the star chart is visible. Because we are at a latitude of nearly 45o (in Toronto)
the part of the star chart that can be seen is enclosed by a curve (the horizon).
Note that on the star chart, the stars of any particular constellation are joined by dashed
lines, thus defining an area of sky associated with that constellation. This is only an
approximate guide to the area that any given constellation covers.
The position of the Moon may be figured out from its phase for any given date once the
sun's position is known.. For example, a full moon occurs 12 hours in Right Ascension
away from the sun. That is, on the opposite side of the sky. (Remember, once around the
sky is 360o or 24 hours of Right Ascension.) The phases of the moon we observe occur
because of the relative position of the moon and the sun. A full moon will always rise as
the sun sets, since we - on the Earth - are directly between the two bodies, allowing us to
see a completely illuminated moon. Once the RA of the moon is determined, based on the
location of the sun, one only has to find where the ecliptic line crosses that line of RA.
Remember, like all solar system objects, the moon is always near the ecliptic.
Planets are also always found near the ecliptic line.
Part B -Mars
Like the Sun and Moon, the planets will also follow the path through the sky that we
call the ecliptic. Planets also undergo something called retrograde motion- this is due to
relative motions of the planets relative to Earth. We move faster than the planet outside
Earth’s orbit of the sun, so as we pass them they seem to slow down, turn around and go
backwards. This is only an optical illusion due to the relative motions, but was quite
confusing to early astronomer who spent a great deal of time trying to explain this
motion. Their task was complicated by the fact that they had come to the erroneous
assumption that the Earth was at the centre of the Universe and everything moved around
us.
Mars is the next planet out from Earth. As it is fairly close to Earth we see it undergo
retrograde motion more often than the other outer planets. Observations of Mars were the
key to Kepler being able to deduce the true nature of the motion of the planets. Here we
use a diagram to show how retrograde motion is actually produced.
Using the table on the next page to plot these points on a diagram that will be handed out
in class. Connect the points in order to give the path of Mars through the sky. Note the
first position has been given. Use the path you have plotted to answer the questions
below. Hand in the plot you have made.
Table of Mars Positions for Retrograde Motion
Date
June 7
June 13
June 19
June 22
June 28
July 4
July 10
July 13
July 16
July 19
July 22
July 25
July 28
July 31
Aug. 3
Aug. 7
Aug. 10
Aug. 16
Aug. 22
Aug. 28
Sept. 3
Sept. 9
Sept. 12
Sept. 15
Sept. 18
Sept. 21
Sept. 24
Sept. 27
Sept. 30
Oct. 3
Oct. 9
Oct. 12
Oct. 18
Oct. 24
Nov. 1
Nov. 8
Right Ascension
Declination
Rising Time
Magnitude
21h 55m 44s
22h 06m 56s
22h 17m 20s
22h 22m 12s
22h 31m 09s
22h 38m 58s
22h 45m 30s
22h 48m 15s
22h 50m 36s
22h 52m 34s
22h 54m 06s
22h 55m 12s
22h 55m 50s
22h 55m 59s
22h 55m 41s
22h 54m 55s
22h 53m 12s
22h 49m 17s
22h 44m 00s
22h 37m 52s
22h 31m 32s
22h 25m 41s
22h 23m 07s
22h 21m 02s
22h 19m 08s
22h 17m 38s
22h 16m 33s
22h 15m 57s
22h 15m 48s
22h 16m 07s
22h 18m 07s
22h 19m 45s
22h 24m 12s
22h 30m 04s
22h 40m 08s
22h 50m 13s
-1550’ 00”
-1509’ 46”
-1433’ 04”
-1413’ 26”
-1347’ 19”
-1324’ 43”
-1309’ 36”
-1305’ 05”
-1302’ 44”
-1302’ 39”
-1304’ 54”
-1309’ 30”
-1316’ 26”
-1325’ 36”
-1336’ 49”
-1349’ 51”
-1409’ 34”
-1442’ 29”
-1516’ 30”
-1547’ 38”
-1611’ 52”
-1626’ 05”
-1628’ 54”
-1628’ 54”
-1625’ 56”
-1619’ 38”
-1611’ 04”
-1559’ 17”
-1544’ 46”
-1527’ 38”
-1446’ 14”
-1422’ 18”
-1328’ 44”
-1228’ 19”
-1055’ 21”
-0928’ 57”
1:11 am
12:56 am
12:40 am
12:32 am
12:16 am
11:55 pm
11.37 pm
11.28 pm
11.18 pm
11.08 pm
10:58 pm
10:48 pm
10:37 pm
10:26 pm
10:14 pm
10:03 pm
09:46 pm
09:21 pm
08:54 pm
08:27 pm
07:59 pm
07:30 pm
07:16 pm
07:02 pm
06:48 pm
06:35 pm
06:21 pm
06:08 pm
05:55 pm
05:42 pm
05:18 pm
05:06 pm
04:44 pm
04:22 pm
02:54 pm
02:31 pm
-0.8
-1.0
-1.1
-1.2
-1.4
-1.5
-1.7
-1.8
-1.9
-2.0
-2.1
-2.1
-2.2
-2.3
-2.4
-2.5
-2.6
-2.7
-2.8
-2.9
-2.8
-2.7
-2.6
-2.6
-2.5
-2.4
-2.3
-2.2
-2.1
-2.0
-1.9
-1.8
-1.6
-1.4
-1.2
-1.0
Questions.
Part A
1. Location of the moon. For the month of October this year, full moon will occur on
the 6th.
a. In what constellations will the sun be located?
b. In what constellations will the moon be located?
c. Roughly what would be the date of the next 3rd quarter and where would it
be located?
2. Given the following positions, identify the specific objects at these locations.
Note, the constellation names are not sufficient for the answers.
a) RA: 03h 44m Dec: +24
b) RA: 13h 40m Dec: +28°
c) RA: 06h 20m Dec: -53°
d) RA: 18h 21m Dec: -16°
3. What are the coordinates of the following bright stars? Which constellation are
they part of?
a. Sirius
b. Vega
c. Antares
d. Capella
e. Alcor
f. Spica
Part B Mars
1. On what date does Mars start to move backward?
2. On what date does Mars start to move forward again?
3. Describe how the motion of Mars appears to change during the retrograde loop.
Name:
Answer Sheet - Exercise # 1
Student Number:
Part A
1. Locations of Moon
a. Sun October 6th _____________________________
b. Moon October 6th ___________________________
c. 3rd Quarter Date and Location ___________________
2. Object identities for various given coordinates.
a)
b)
c)
d)
3. Star locations and Constellations
Star
R.A
Dec.
Constellation
.
Name:
Answer Sheet - Exercise # 1
Mars
Hand in Chart
1. Backward motion begins?
2. Backward motion ends?
3. Brightest part of loop?
Student Number:
.