Recognizing the planets in the night sky is a real treat. Being able to identify them creates a familiarity. You look up in the sky and there is one of your friends!


Several of the planets are brighter than most of the stars, so the recognition is usually easy. Once you know the constellation patterns, the identification of planets is simple. Either you see an extra "star" that does not belong in the constellation or, if you know which constellation the planet is in, you have a general idea of where to search for it. Because the planets have orbits in nearly the same plane as the Earth's, they are generally found along the Ecliptic in the Zodiacal Constellations.


All ancient civilizations identified five wanderers in the night sky: Mercury, Venus, Mars, Jupiter, and Saturn. In addition, there are currently two other planets – Uranus and Neptune. The motions of the planets fall into two categories, Inferior and Superior Planets, and we shall investigate these two types.


Planetary Motion

Because Mars, Jupiter, and Saturn have larger orbits than the Earth's, they are known as Superior Planets. Their farther orbits make it possible for them to be seen anywhere along the Ecliptic. Depending upon their orbital positions, they can be seen at any time of the night and at any angle away from the Sun. These three do not experience the phase changes as do Venus and Mercury, for their appearances are always near "full." They do have significant brightness variations, though, because of their large changes in distance from the Earth. Mars has a reddish tint, Jupiter glows white like a pearl, and Saturn has a yellowish hue.


The diagram below shows the orbits of the Earth, Mars, Jupiter, and Saturn to scale. When the superior planets are on the other side of the Sun on the Earth-Sun line, they are at Conjunction. Opposition is the location directly opposite from the Sun as seen from the Earth. This is the time of the planet's closest approach to us.


At first, one would rightly think the motion of these planets would be uniform and constant across the sky. After all, with respect to the stars they are always moving counterclockwise around the Sun. However, as seen from the Earth, each one periodically reverses its direction on the sky for days to months! Later, they change directions again and proceed on as if nothing has happened. A prescribed number of months later, the reversal occurs again.


Nothing has altered their regular orbits as viewed from above the Solar System. Their Retrograde Motion is an observational phenomenon due to the Earth's motion around the Sun. The nearer a planet is to the Sun, the faster its orbital speed. For example, as the Earth catches up with Mars near opposition, the Earth's faster speed means it travels a greater distance in the same amount of time. Against the distant stars, Mars appears to move backwards, even though it has not changed its motion. [The same phenomenon is seen with cars driving on the highway. As a faster car approaches and overtakes a slower one, the slower car appears to be moving backwards against the distant terrain.]



Jupiter and Saturn, along with Uranus and Neptune, are quite different from the hard, rocky terrestrial planets, Mercury, Venus, Earth, and Mars. Rather, these giant worlds are primarily gaseous. There are no hard surfaces, just gas clouds that extend down to the core. Jupiter is the largest and most massive planet in the Solar System. It is also the fastest rotator, for one day is just under ten hours! This rapid rotation causes it to be slightly flattened, and it stretches the weather patterns (winds) into long continuous bands.


Saturn is best known for its rings. Without them, it would be a rather dull, yellowish world, whose stripes are much less distinct than Jupiter's. But the rings make it one of the most memorable views in the night sky, although a small telescope is required. The rings are not continuous sheets of material but are instead made up of billions of tiny particles, each in orbit about Saturn as a moon. The particles range in size from snowflakes, to baseballs, to a few trashcan-sized pieces. However, the rings are only about ten yards (ten meters) thick! Saturn has 53 known moons, and the largest one, Titan, is shrouded in an orange gaseous atmosphere.



The Naked-Eye Sky (copyrighted) by James Sowell, 2013