why do stars only come out at night

The twinkling of stars in our night sky is down to the EarthБs atmosphere. As light from the stars travels towards Earth it can easily move in a straight line, but once it starts travelling through the EarthБs atmosphere it gets bounced around in different directions by the particles in the air. For astronomers this twinkling can be a huge problem, especially when theyБre trying to accurately image a particular star. The more the star twinkles, the blurrier the image will be. If telescopes are launched into space then all images can be taken without EarthБs atmosphere in the way, but this is expensive. It is possible to use a system called Бadaptive opticsБ, when tiny motors alter the surface of the telescopeБs mirror to correct for this blur caused by the air.


Answered by Megan Whewell, Education Team Presenter for the National Space Centre. Image courtesy of NASA Keep up to date with theб latest бreviews in All About Space Бб available бeveryбmonthбfor just бе4. 99. Alternativelyбyou canбsubscribeб
б for aб fractionб of theб price! Tags:, Answer 4: This question can be partially answered by several effects. First, the human eye is sensitive to light over an enormous range of intensities. Typical "bright" stars are actually very dim compared to the brightness of the sun -- about a trillion 10^12 times dimmer. The brightness of objects illuminated by the sun typically 10 to 100 times dimmer than it is -- but that still makes the an enormous problem for the eye. (It has at least two mechanisms for light accommodation-- the iris which simple stops a fraction of the light like a camera lens, and chemical changes which enhance the cells particular to night vision. -- this takes some 10 to 20 minutes and unless you walk rapidly from dim to bright or vice versa you won't notice it. ) So the first problem is that your eye -- to allow you to see, will reduce its sensitivity when there is bright light present.


Secondly, the atmosphere of the earth has lots of fine particles of dust which are so small they never settle to the ground. These dust particles scatter a small part of the light from the sun in random directions, and the mechanism for scattering works better for shorter (bluer) wavelengths.


This is why the sky is blue and why the sun seems to redden as it sets -- it has a longer path in the air -- so more of the blue light is lost, then green. etc. By the way, clouds are made of much bigger bits, so they scatter visible light fairly evenly -- making them appear white. (But not to even longer light like infrared -- which can peer through clouds). Both processes make seeing stars in the daytime very hard. However, if you happen to be in a deep well, with dark walls so that you only see a small bit of sky, and the rest of you visual field is dark, you might see some bright stars during the day. So, do you think that the astronauts can see the stars during the day from the moon?

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