Celestial Motion of Planets – Orbits in the Ecliptic Plane
The planets fall into two groups relative to Earth’s location:
- Outer Planets : Sun–Earth–Planet — Mars, Jupiter, Saturn, Uranus, Neptune, [pluto]*
- Inner Planets : Sun–Planet–Earth — Mercury, Venus
*As of August 24, 2006 – The International Astronomical Union reclassified Pluto as a ‘dwarf planet’. It is regarded as one of the largest members of distant objects called Trans-Neptunian Objects (objects orbiting the Sun at a greater average distance than Neptune. Also note that Pluto’s orbit does sometimes slip inside of Neptune’s due to its ellipticity.
Outer or Superior Planets (beyond Earth) :
- Mars – naked eye visible when available
- Jupiter – ditto –
- Saturn – ditto –
- Neptune – need telescope and time of transit
- Uranus – ditto
Opposition – Best Time to view Outer Planets
Opposition occurs when the angle Sun-Earth-Planet is is a straight line, and the planet is on the opposite side of the Sun. Opposition of the planet means the planet receives full Sun illumination as seen from Earth – like the full moon. We get to see the whole disk.
Notes: The relative size of the disk of the planet depends on how close to the earth (in AUs (astronomical unit = Sun-Earth units) it is. The most favourable oppositions occur at the smallest distance between the Earth and Mars – we can see this around 2020.
This diagram provides graphical representation of the variation of the Martian disk. Note also that the Martian Disk will never be larger than the lunar one because it is so much further away. The moon is only a small fraction of an Astronomical Unit distance :.0024 AUs .. much much closer – appearing much much bigger!
The Motion of the Planets – Night After Night
- The farther the planet lies from the Sun, the slower it moves around the Sun.
- When the Earth and another planet pass each other on the same side of the Sun, the apparent retrograde loop occurs. This is an observational phenomena, not an actual loop in space. It is caused by the relative angular speeds of the Earth and the outer plane.This shows up as a change in direction from one night to the next when we make our observations.
- Retrograde Motion Described…
Then for a few nights, the planet goes into retrograde (moves in the opposite direction from the night before) as the earth in its closer orbit to the Sun speeds past it. (This is the case for Outer planets – for Inner planets, the Inner planet speeds past us)
Retrograde motion happens when the angular speed of planets appears to slow down (outer) or speed up (inner) when planet and Earth are moving in the same direction, around the sun, and the earth ‘overtakes’ the planet, like a faster car on a highway.
- Here is an animation of Mars (outer planet) as it approaches opposition. We observe the motion as West to East (R-L) with respect to earth until we come closer to opposition. http://millstonenews.com/millstone/wp-content/uploads/2013/04/opposition.gif
Inner or Inferior Planets
At greatest Eastern Elongation, the inferior planet appears higher and higher in the Western Sky and appears as an evening ‘star’ .It follows the Sun and is visible after sunset. It then moves closer to the horizon at sunset as it sweeps through the sun, disappears in the glare of the Sun.
This disappearance occurs at Inferior Conjunction. The planet then reappears as a pre-dawn morning ‘star’, preceding the sun (west of) before sunrise so we can see it while still dark.
When Venus is at Inferior Conjunction, it’s apparent diameter is much larger because it is only at the distance of Earth’s Orbit – Venus’s orbit (roughly 1 AU – .7 AU)
When Venus is at Superior Conjunction, it appears much smaller because it is opposite the Sun at a distance of the Earth’s Orbit + Venus’s Orbit (roughly 1 AU + .7 AU)
Courtesy Tenho Tuomi, RASC member Saskatoon ttp://www.lex.sk.ca/astro/
In October, planet Venus was very very very bright at Sunset: Here’s the post about that: http://www.mvc.on.ca/venus-bright-sunset-october-2013/