mic wrote:Nice bp, very nice,
Yes Gordon is spot on Venus & Mercury, you know it's not a star bp if you go out and it doesn't twinkle at you, it will have a steady light, as it is a planet reflecting light from the sun back to us and not making it's own light like a star. Or our Sun.
Mic.
you didnt quite get it right there with the twinkling explantion mic
Planets generally dont twinkle (but will if the air is unsteady enough) not because they are reflecting sunlight, but because they are a non-point in the sky, ie they have a diameter, even if you cant resolve it with your naked eye.
Warmer pockets of air are rising up through the atmosphere day and night. Daytime we call it heat haze, the night time equivalent is scintillation, commonly known as twinkling. The apparent diameter of these air pockets when viewed from the ground is very small, but larger than the apparent diameter of all stars. The disk of a planet generally is large enough that several to many of these pockets of rising warm air will be across the disk, so the effect is averaged out. They act like small lenses, refracting and dispersing the starlight (or reflected sunlight from a planet), causing the starlight to deviate a noticeable amount- which we see as the stars jiggling around and changing colours. The effect is much more obvious near the horizon because we are looking through a much greater thickness of atmosphere. Planets seen low down usually twinkle too.
I've seen Venus very low over a distant horizon appear as 3 seperate images through a telescope, red, green abd blue! To the naked eye they were merged into a twinkling point with lots of colour.
Gordon