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Brightness and Size

Outside of the Sun, Moon and, perhaps, the odd Comet, Venus is the brightest object in the sky.

It is also, uniquely, the object that varies so much in size as it is about 7 times further from the Earth when it is on the far side of the Sun than it is when on the nearside and ironically this means that Venus is brighter in the sky when close to the Earth than when distant, despite it's being "full" when distant.

If we assume that the light from Venus falls off as the square of its distance from Earth, the fact that it is only 1/7th the size when distant results in 1/49th of the light reaching Earth than if it were close. This means that when close and in crescent form, Venus only needs just over 2% of its surface to be illuminated in order for it to appear brighter than when distant.

We show the scaled size of Venus in the upper right of the display. You can watch its variation in size and phase by clicking on the "Simulate" button.

Retrograde Motion

Unlike the stars, planets are not in fixed positions on the celestial sphere. They tend to move eastwards against the backdrop of stars. However, occasionally they pause and for a short while move in a westward direction before pausing again and continuing their eastward movement. This is called Retrograde Motion.

Our software detects when Venus is in retrograde and reports this as a red message in the right margin of the simulation. It also draws the line between the Earth and Venus in red at the same time.

You can watch this phenomenon by clicking the "Simulate" button and allowing the process to run continuously. You may also want to select the "Geocentric view" that puts the Earth at the centre of the simulation screen and to "Hide" the orbits.

You will see that the retrograde motion occurs when Venus is at its closest to the Earth when, due to its faster motion around the Sun, and in the opposite direction from the Earth at this point, Venus goes west faster than the Earth is going east.