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Asia-Pacific
Forum on Science Learning and Teaching, Volume 14, Issue 1, Article 9 (Jun., 2013)
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If we know the rise or set time of the planets, it is possible and easy to use the planisphere to locate the planets. The idea is simple. First, the planets are always near the ecliptic because the inclinations of the orbital planes of the planets are only a few degrees from the ecliptic (JPL Solar System Dynamics Group, n.d.) .
Table 1. Inclinations of planetary orbits
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Inclination of planetary orbits (degrees)
7.00
3.39
0
1.85
1.30
2.49
0.77
1.77
In the planisphere, the ecliptic is represented by an off-center circle on the circular star chart (Figure 2). Some planispheres may not have the ecliptic drawn but the users can roughly estimate the ecliptic because the ecliptic runs through the zodiac constellations. Since the planets must appear near the ecliptic plane, the issue is to determine at which points on the ecliptic path the planets lie.
Figure 2. Circular star chart of the planisphere with ecliptic shown in dotted line.
The part of the sky being visible at any particular time and date appears in an oval window of the cover overlaid on the circular star chart (Figure 3). The oval window represents the horizon of the observation location. The oval boundary intersects the ecliptic at two points, one at the east horizon and the other at the west horizon (Figure 4).
Figure 3. An oval shape window in the cover to show the part of the sky at a specific time and date
Figure 4. The oval shape horizon boundary intersects with the ecliptic at two points, Virgo (Vir) in the east and Pisces (Psc) in the west
If we set the planisphere at the rise or set time of the planets, the intersecting points are the positions of the planets. The example in Figure 4 shows that the planet risen on the east horizon is in Virgo (Vir) and the planet set on the west horizon is in Pisces (Psc). Figure 5 shows these two intersecting points in a topocentric view. Therefore if we know the rise and set times of the planets, we can determine the location of the planets.
Figure 5. Topocentric view of the celestial sphere at a location in the northern hemisphere. The ecliptic, running through the twelve constellations of the zodiac and represented by dotted line, intersects with the horizon at two points
The next thing we need to know is the rise and set times of the planets. Fortunately this information is easily accessible. The rise and set times of the planets are published by local observatories. They are also available at these websites.
(a) Astronomical Applications Department of the U.S. Naval Observatory
Rise/Set/Transit Times for Major Solar System Objects and Bright Stars
(b) HM Nautical Almanac Office
http://astro.ukho.gov.uk/websurf/
Rising and setting times of Sun, Moon or planet
The rise and set times of planets may also be presented in a graph such as the one given by the Hong Kong Observatory (2013).
