The Way the Sky Appears to Move: Diurnal Motion and Parans
Due to the rotation of the earth on its axis, the stars and planets will appear to move in the sky - travelling from rising, to culminating, to setting, and then to rising again. This movement is called diurnal movement, and one complete circle gives us, of course, one day.

Watch this animated celestial sphere, as it moves through a period of time. In this example, looking from London, the movement is from just before dawn on a day in Oct, 1961 to a time just before local noon on the same day.

Note the North Pole (the place where all the blue lines intersect), and that it does not move.

The North Pole will always be in this position for London, a position that reflects London's latitude of 51° north. For the elevation of the pole above the horizon is always equal to the latitude of the location. If we moved our location to a place that was 20° North in latitude, then the pole, in this figure, would be closer to the horizon, (the green line) with an elevation of only 20° above the horizon.

The poles, north and south, are important because they define the axis on which the celestial sphere, and therefore all the stars, rotate.

You will also notice that some stars, which are close to the pole, are not going to dip below the horizon line during the course of diurnal motion (day's movement). These stars are called Circumpolar, and will be visible every night for this location.

In contrast to this, when observing the area of the South Pole (which, located opposite the north pole, is not visible in the figure ) you can see that stars in this area of the sphere will never rise above the horizon. These stars will never be visible from London.

There are, however, other stars, such as Vega, Spica, Arcturus, and Antares, to name just a few, which do rise over the horizon and, at some stage of their diurnal movement later in the day, they will culminate, set, and also travel over the nadir. These stars will be visible from London at different times of the year.

The Sun and the planets, however, present no such variation. Being on or close to the ecliptic, they will all, during the course of the diurnal movement (the course of the day), rise over the horizon, culminate on the MC (which is marked in RED), set, and later will also travel over the nadir.

Planets that rise at the time as a star rises, so that both are on the line of the horizon at the same time, are considered to be connected. In this animation, Venus rises as Vega and Arcturus also rise, so Venus would be in paran with Vega and Arcturus.

As you watch this animation, you will also notice some planets will culminate as some stars are rising. You can see, for example, that when Venus reaches the MC (marked in red on the ecliptic), Altair, the alpha star of the Eagle, rises.

In summary a star is said to be in paran with a planet when a the planet is on any of the four points of rising, culminating, setting, or on the Nadir, and at the same moment in time, for that location, the star is also on ANY of the same four angles or points.

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