Planetary Motion from an Earthly Perspective

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 noticeableover tens of thousands ofyears

2. Planetary Motion • Within the Solar System ourperspectiveon the planets changes quickly. • 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.

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

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.