what is the distance from the sun to alpha centauri
Alpha Centauri Alpha Centauri (α Centauri, abbreviated Alf Cen, α Cen) is the closest star system to the Solar System, being 4.37 light-years (1.34 pc) from the Sun. It consists of three stars: Alpha Centauri A (also named Rigil Kentaurus[13]) and Alpha Centauri B, which form the binary star Alpha Centauri AB, and a small and faint red dwarf, Alpha Centauri C (also named Proxima Centauri[13]), which is loosely gravitationally bound and orbiting the other two at a current distance of about 13,000 astronomical units (0.21 ly). To the unaided eye, the two main components appear as a single point of light with an apparent visual magnitude of −0.27, forming the brightest star in the southern constellation of Centaurus and is the third-brightest star in the night sky, outshone only by Sirius and Canopus.
Eratosthenes Eratosthenes calculated the circumference of the Earth without leaving Egypt. He knew that at local noon on the summer solstice in Syene (modern Aswan, Egypt), the Sun was directly overhead. (Syene is at latitude 24°05′ North, near to the Tropic of Cancer, which was 23°42′ North in 100 BC[16]) He knew this because the shadow of someone looking down a deep well at that time in Syene blocked the reflection of the Sun on the water. He then measured the Sun's angle of elevation at noon in Alexandria by using a vertical rod, known as a gnomon, and measuring the length of its shadow on the ground.[17] Using the length of the rod, and the length of the shadow, as the legs of a triangle, he calculated the angle of the sun's rays. This turned out to be about 7°, or 1/50th the circumference of a circle. Taking the Earth as spherical, and knowing both the distance and direction of Syene, he concluded that the Earth's circumference was fifty times that distance.