MAPPING THE POSITIONS OF THE PLANETS
Materials: Equatorial Map(SCI), ruler, ½ cm graph paper (attached).
Keep this portion in your notes.
The positions of celestial objects as viewed from the earth are called geocentric positions. Such
positions are specified by right ascension and declination. You learned about these coordinates in a previous
investigation. The positions of objects seen from the sun’s point of view are termed heliocentric positions.
One set of coordinates used to specify these positions are heliocentric latitude and heliocentric longitude.
Heliocentric latitude is the position of an object, expressed in degrees, above or below the ecliptic.
Heliocentric longitude is measured from the vernal equinox, eastward, along the ecliptic.
Notice that heliocentric latitude and longitude are very similar to right ascension and declination !! The
only difference is that the former coordinates are measured along the ecliptic, while the latter, are
measured along the celestial equator.
THE GEOCENTRIC POSITIONS OF PLANETS
The geocentric positions of planets and stars can be located on the equatorial star map, SC1. The
straight, dashed line across the middle of this map denotes the celestial equator. The wavy, dashed curve
represents the ecliptic.
On the map, hour circles are lines drawn perpendicular to the celestial equator. Five such lines are
indicated on the map (two on either side and three in the middle.) These lines are divided into one-half degree
intervals of declination, ranging from zero degrees on the equator to 60 degrees above (+) and below (-) the
equator. By placing a ruler at right angles to the equator, you can represent the hour circle through any object
on the map.
The celestial equator and ecliptic intersect in the points knows as the equinoxes! On the map, the vernal
equinox is right in the middle, while the autumnal equinox is shown at both ends of the celestial equator. If
you were to make a cylinder of the map, the two positions at either end would coincide. Right ascension is
measured from the vernal equinox eastward. The map can be used outside at night by holding it in front of
yourself as you stand facing south. Note that this means that west is on the right of the map and east is on the
left. Right ascensions are marked on the celestial equator and also on four lines parallel to the equator. The
smallest division on these lines is 4 minutes of time.
1. Plot the positions of the sun, moon, and planets for the first month on the equatorial star map. Use
the data from Data Table #1. Be sure to label the positions for future reference. Use a pencil rather
than a pen.
2. a) Is Mercury to the east or west of the sun? __________________________
b) Would you expect to see Mercury in the early morning or early evening hours? ____________
c) Does Mercury rise before or after the sun rises? __________________
3. Repeat question #2 for the planet Venus, Mars, Jupiter, and Saturn.
Venus Mars Jupiter Saturn
a) ___________ a) ________ a) __________ a) ___________
b) ___________ b) ________ b) __________ b) ___________
c) ____________ c) _________ c) __________ c) ___________
On the next clear evening go outside and try to identify those planets that are visible.
5. Plot the positions of the sun, moon, and planets for the second month on the equatorial star map
(this is a three month time lap from the first month you plotted in question #1). Use the data from Data
Table #1. Be sure to label the positions for future reference. Use a pencil rather than a pen.
6. Relative to the stars, what is the direction of movement for the planets (cardinal direction)?
7. What causes this motion? _____________________________
8. What part of the celestial sphere do the planets appear to move through as seen from the earth?
9. Have any of the planets experienced retrograde motion? ________________
If yes, which one? ___________________
Usually planets appear to move eastward among the background stars, their right ascensions increasing
with time. Every so often, however, planets reverse their eastward motion and seem to move backward with
decreasing right ascension. This westward motion is called retrograde motion (see figure 1).
Procedure for part 2:
Data Table #2 lists the geocentric positions of Mars for the ending of 1977 and the beginning of 1978.
Plot these positions on a piece of ½ cm graph paper, marking the dates of each plotted position and the direction
of motion. The horizontal axis should be divided into minutes of time (each square represents 3 minutes of
R.A.) with right ascension increasing to the left (east). The vertical axis should be divided so that each square
represents 10 minutes of arc, with declination increasing upward (north).
Figure 1: Retrograde Motion
10. About what date does retrograde motion start? ________________________
11. About what date does retrograde motion end? ______________________
12. When a planet retrogrades it appears brightest. Explain why this occurs?
Data Table #1 (this data should be graphed on your SC1 chart)
March 14th, 2005
Planet/Sun Right Ascension Declination
Sun 23h 39m -02o 10’
Mercury 00h 41m 07 o 12’
Venus 23h 27m -05 o 05’
Mars 19h 52m -21o 46’
Jupiter 13h 02m -04 o 59’
Saturn 07h 28m 21 o 59’
June 14th, 2005
Planet/Sun Right Ascension Declination
Sun 05h 34m 23o 18’
Mercury 06h 33m 25o 09’
Venus 07h 00m 23o 58’
Mars 00h 11m -01o 19’
Jupiter 12h 35m -02o 20’
Saturn 07h 50m 21o 10’
Data Table #2 (this data should be graphed on the attached graph sheet)
Date Right Ascension Declination
Oct. 27, 1977 8h 10m 21o 24’
Nov. 02 8h 21m 21o 00’
08 8h 30m 20o 42’
15 8h 40m 20o 24’
24 8h 50m 20o 06’
Dec. 14 9h 00m 20o 18’
31 8h 51m 21o 36’
Jan. 01, 1978 8h 50m 21o 42’
06 8h 44m 22o 18’
09 8h 40m 22o 36’
16 8h 30m 23o 30’
22 8h 20m 24o 06’
28 8h 10m 24o 36’
Feb. 03 8h 00m 25o 00’
11 7h 50m 25o 24’
25 7h 40m 25o 24’
Mar. 05 7h 39m 25o 06’
10 7h 40m 24o 54’
25 7h 50m 24o 00’
Apr. 03 8h 00m 23o 18’
10 8h 10m 22o 42’
17 8h 20m 21o 54’
23 8h 30m 21o 18’
29 8h 40m 20o 30’
May 04 8h 49m 19o 48’
10 9h 00m 19o 00’