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There are some key words that you need to know for circles: radius, circumference, diameter, sector, segment, tangent, chord and arc. In this video you will discover what they all mean.

This video discusses the topic of circles.

These two short online tutorials discuss how the earth rotates and then the earth's revolutions.

In the first URL, the YouTube video explains why earth has spring, summer, autumn and winter seasons. The second URL summarizes the key takeaway points.

This video explains the equation of a circle.

This online resource introduces the geocentric and heliocentric models. Exoplants, planets and their motions, planetary orbits, the role of gravity and how the solar system was formed are also covered. 

This fascinating video details the journey that a group of friends made in Nevada. Their aim was to illustrate the solar system to scale.