Longitude represents angle east or west of the Prime Meridian, which since 1884 has been taken to be the meridian running through the site of the Royal Observatory at Greenwich, England. The idea is that a planet is a wandering star, that is, a star that appears to move through the Zodiac constellations as do the sun and moon. Without the aid of a telescope it is not easy to distinguish between a planet and a star. Indeed, to the naked eye, a planet appears to be little more than a bright point of light, having much the same appearance as any of the stars. The planets appear to have the appearance, however, of rather bright stars, and Venus in particular is at times far brighter than Sirius and can be the brightest object in the sky other than the sun and the moon.
Twinkling is produced by unsteadiness in the atmosphere.
It is the larger apparent angular diameter of the planet that reduces the twinkling phenomenon. Of course if you have access to an ephemeris for the planets, you can read off the equatorial coordinates of each of the planets and locate them on your sky chart and identify them in the sky. There are five planets that can be seen at one time or another without the aid of a telescope. The names that we have inherited for these planets derive from the names of Roman deities. Therefore the declination of Polaris has very nearly that value.