Today is Christmas in the Christian world. The full moon also happens to fall on this day. A full moon occurring this time of year is called a cold moon as it is the beginning of winter in the northern hemisphere (though, right now, much of the eastern US is actually warm rather than cold).
The strange thing is that the full moon has not been on Christmas Day since 1977 and will not occur again until 2034.
Nothing else is unusual about the full moon today, but just the date it happens to fall on.
25 December 2015
21 December 2015
Twinkling Stars and Static Planets
Twinkle, Twinkle Little Star
How I wonder what you are.
Everyone remembers this nursery rhyme. The question is why do stars twinkle? And how come planets do not?
The short answer is that it has to do with the apparent diameter of the star and the planet. Stars are generally so far away that they have no apparent diameter, so that when light from the star enters our atmosphere, the light is easily diffracted due to different pockets of air. Since the width of the light is so small, these diffractions make it seem as if the star's light is constantly blinking in and out as the light is diffracted away from our eye as the light travels through the atmosphere.
For planets, however, they do have an apparent diameter, albeit much tinier than that of the Sun or the Moon. But this size is enough that when light from the planet travels through different pockets of air in the atmosphere, only a small portion of the light is diffracted away from our light of sight, so the brightness of the planet does not waver, and therefore does not twinkle.
How I wonder what you are.
Everyone remembers this nursery rhyme. The question is why do stars twinkle? And how come planets do not?
The short answer is that it has to do with the apparent diameter of the star and the planet. Stars are generally so far away that they have no apparent diameter, so that when light from the star enters our atmosphere, the light is easily diffracted due to different pockets of air. Since the width of the light is so small, these diffractions make it seem as if the star's light is constantly blinking in and out as the light is diffracted away from our eye as the light travels through the atmosphere.
For planets, however, they do have an apparent diameter, albeit much tinier than that of the Sun or the Moon. But this size is enough that when light from the planet travels through different pockets of air in the atmosphere, only a small portion of the light is diffracted away from our light of sight, so the brightness of the planet does not waver, and therefore does not twinkle.
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