1 Planetary Motion from an Earthly Perspective Stars appear “fixed” from night-to-night providing the familiar background of the constellations and asterisms. We see the same star patterns that were visible to ancient civilizations. The stars ARE moving but they are awfully far away.  The changes in star patterns becomes noticeable over tens of thousands of years noticeableyears

2 Planetary Motion Within the Solar System our perspective on the planets changes quickly.perspective Even from day-to-day we can see other planets move with respect to the backdrop of distant stars.

3 Planetary Motion Within the Solar System our perspective on the planets changes quickly. Even from day-to-day we can see other planets move with respect to the backdrop of distant stars – the word planet comes from the Greek for “wanderer”.

4 Characteristics of the Classical Planets Five planets – Mercury, Venus, Mars, Jupiter, and Saturn, are visible to the unaided eye. All of them are bright compared with the majority of the stars. Some – Venus, Mars, and Jupiter – can outshine the brightest stars in the sky.  Venus is the brightest of all.

5 Characteristics of the Classical Planets The planets can always be found near the Ecliptic in the sky. That means that they will always be within 20 degrees or so of the celestial equator.  Face South and trace the celestial equator from due East to due West and there's a good chance you will see a planet.  Face North and you are unlikely to see a planet.

6 Characteristics of the Classical Planets The planets can always be found near the Ecliptic in the sky. Planets will often pass and even “occult” one another. The Moon follows a simliar path.

7 Characteristics of the Classical Planets The planets can always be found near the Ecliptic in the sky. Planets will often pass and even “occult” one another. The Moon follows a similar path.

8 Characteristics of the Classical Planets The brightness of a planet will vary over time. Mars, for example, sometimes outshines the brightest stars, at other times it becomes so faint that it looks like a typical star on the sky.

9 Retrograde Motion Planets tend to drift eastward against the background of stars, but occasionally they will stop, back up, and continue on eastward again. This reversal is called “retrograde motion”.

10 Inferior vs. Superior Planets The Earth is the third planet out from the Sun in the Solar System. Mercury and Venus follow orbits inside the Earth's Mars, Jupiter, and Saturn orbit outside the Earth's orbit. The behavior of Mercury and Venus is strikingly different from Mars, Jupiter, and Saturn as a result.

11 Inferior Planets – Mercury and Venus Since their orbits lie entirely within the Earth's orbit we always see them close to the Sun in the sky. They can only be “evening” or “morning” objects.

12 Inferior Planets – Mercury and Venus Since their orbits lie entirely within the Earth's orbit we always see them close to the Sun in the sky. They can only be “evening” or “morning” objects. Inferior planets will never be seen in the midnight sky. Mercury is notoriously difficult to see since it is never very far from the Sun at all.

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14 Superior Planets – Mars, Jupiter, Saturn... These planets have orbits outside the Earth's orbit. They can never be between the Earth and Sun. Unlike an inferior planet they can be opposite the Sun in the sky.

15 Planetary Configurations As the planets orbit the Sun, occasionally they reach interesting geometrical alignments with the Earth and Sun. The simplest of these are Opposition – the planet and Sun are in opposite directions in the sky.  Opposition is a terrific time to see a planet. It rises when the Sun sets and is high in the sky at midnight.  Opposition is also when a planet is closest to the Earth. It's brighter and more interesting to see in a telescope.  Only superior planets can be at opposition. Conjunction – the planet is in line with the Sun as seen from Earth  Conjunction is the worst time to see a planet because it is lost in the Sun's glare.  Inferior planets have both Superior conjunction (on the far side of the Sun from Earth) Inferior conjunction (between the Sun and Earth)  Superior planets also can be found in Superior Conjunction

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17 Planetary Configurations As the planets orbit the Sun, occasionally they reach interesting geometrical alignments with the Earth and Sun. Greatest elongation is the time when an inferior planet is seen at its greatest angular separation from the Sun. Greatest elongation is the ideal time to view Mercury or Venus.