The use of planisphere to locate planets

There is a common misconception to most people that planets could possibly appear anywhere in the night sky. One of the causes of this misconception may stem from the mixing up of stars and planets because both types of objects appear as points of light to the naked eyes. Since stars appear across the sky, most people also think that the planets could be anywhere in the sky. The other cause is that most people do not realize the orbits of the planets are very close to the ecliptic, i.e. the path of the Sun which appears to move annually in the celestial sphere (Figure 1). Thus the planets are confined to appear only in places which are near the ecliptic plane.

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Figure 1. The apparent annual motion of the Sun on the celestial sphere

To help students correct the misconception and develop the concept that the orbits of planets are coplanar, using plainsphere to locate planets is a practical learning activity which can promote a deeper understanding of the concept. The property that orbital planes of the planets are coplanar is not coincident and has its origin in the formation of the solar system (Goldreich, Lithwick, & Sari, 2004).

Planisphere has long been used by many novices or even experienced observers to identify constellations and stars in the night sky (Hughes & Stott 1995). It is convenient, inexpensive and easy to use. Even beginners can use it skillfully with little training. Though the planisphere is suitable for finding constellations and bright stars, it does not show where the planets appear in the sky because the positions of the planets are not fixed on the celestial sphere. This deficiency causes inconvenience to observers because the planets are one of the interesting observation objects and they are also very appealing to novice observers. To extend the application of planisphere in finding planets, some planispheres provide the positions of the planets in tables, showing the constellation or the coordinates of each planet within a certain range of years on the backside of the planisphere (e.g. Philip's planisphere) or in a separate booklet (e.g Edmund Scientific planisphere). Thus most people think that planisphere cannot be used to locate the planets without such information ("Planisphere", n.d.).

Electronic star map, which is available as app in mobile devices, overcomes this drawback and makes stars and planets identification much easier. Despite an increasing popularity of star map app for mobile phones and tablets, planisphere still has its edge over electronic star map in terms of price, simplicity and pedagogical value.