MOTIONS IN THE SOLAR SYSTEM
All celestial bodies rotate on their axes and revolve in their orbits, the solar system as a whole moves with the sun as it travels through space. All the planets rotate from west to east, including the earth.
The subject of the motions of celestial bodies in their orbits, the speed of a celestial body in its orbit varies.
The earth's only natural satellite is the moon and is an average distance of 239,000 miles from earth.The diameter of the moon is roughly 2,160 miles. Its period of revolution is about the earth and its axial rotation are the same 27 1/3.
The sidereal period of revolution for Venus is only 224.7 days; for the earth it is 365.2 days. The maximum brilliance, as observed from the earth, occurs about 36 days prior to and after the inferior conjunction of the two planets, at this time it approaches a magnitude of -4.4. The minimum magnitude is about -3.3. For a portion of the time, Venus can be used for daytime celestial fixes (I have done this 3 times) in combination with lines of position acquired by sun or moon observations. Horizontal parallax is a small factor in planet observations.
Other planets may be observed during daylight hours, subject to the telescope used and atmospheric conditions. Jupiter varies in magnitude from -1.4 to -2.5, Mars from 1.6 to -2.8, Mercury from 1.1 to -1.2. These bodies compare very favorably in brilliance with the 57 selected stars which range from faint Zubenelgenubi, with a magnitude of 2.9, to the brightest star, Sirius, with a magnitude of -1.6. The light from a planet is usually somewhat steadier than that from a star. The full moon has a magnitude varying slightly around -12.6; the magnitude of the sun is about -26.7.
Like the moon, the sun varies in apparent size, depending on the earth's position in its elliptical orbit. Geocentric parallax, a value much smaller than that involved in moon observations, due to the much greater distance of the sun, amounts to approximately 0.1' between altitudes 0° and 65°. It is interesting to that the majority of the principal planets have satellites, or "moons," rotating about them. These satellites, like the planets themselves, are relatively cold bodies, and shine only due to the light reflected from them.
Mercury is the smallest planet about 3,100 miles in diameter, over 3,000 minor planets have been discovered but there are still thousands more
