As students continue their monthly plotting of the planets along the ecliptic they should start to notice differences between inner and outer planet orbital motions, and their relative position or separation from the Sun. Both inner and outer planets have direct eastward motion, as well as retrograde motion. Inner planets Mercury and Venus, however, are limited in their separation from the Sun because they orbit between the Earth and the Sun. An inner planet can never be at opposition, on the opposite side of the Earth from the Sun, for example. Conversely, an outer planet such as Mars can be at opposition but never at inferior conjunction (come between the Earth and the Sun). While plotting the planet positions, students should also note that the Sun is continuously moving, following its apparent path along the ecliptic. They should watch for the day when the Sun reaches the coordinates of right ascension (RA) 18 hours and ?23.5 degrees declination. When the Sun is at this location, Northern Hemisphere autumn will end and Northern Hemisphere winter will begin. The opposite seasons will end and begin for the Southern Hemisphere.