Why is the sky blue?

Why is the sky blue?

In the first approximation the skies on planet Earth are deep blue. Although it seems quite obvious (because we are used to it), it is not so at all. In fact, the sky over lunar surface is rather black, and the Martian sky photographed by numerous space probes is more or less red and yellow. So why the sky is almost always blue on Earth?

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What is the oldest known object in the Universe?

What is the oldest known object in the Universe?

This question seems to be very simple, but in fact is as complex and dangerous as only a cosmological problem can be! First of all it requires a very careful defining of almost every word included: “is”, “the oldest”, “known”, “object” and finally “in the Universe”. Let’s start.

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How many stars can you see at night?

How many stars can you see at night?

There are billions of stars just in our galaxy and it is estimated that in the visible Universe, there are about 200 – 300 billion large galaxies. It gives a HUGE amount of stars – approximately 10^24. But how many of them can we observe with the naked eye? Of course, directly we can only see the stars of our own galaxy, Milky Way. This is because of the large distance to other galaxies which we only observe as faint clouds in the sky. In fact, only few galaxies are visible to the naked eye in Earth’s sky, including famous Andromeda Galaxy M31 and both Magellanic Clouds. But using the powerful optical telescopes we can separate distant galaxies into individual stars.

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