Angles during Half Moon
One of the methods we use today to calculate the distance to the Sun is by Venus transition proposed by James Gregory (1638-1675). That is why scientists get so excited for the Venus transit. During a Venus transition, a rare event that happens twice in every 243 years, Earth, Venus and the Sun line up in a straight line. Thanks to Johannes Kepler, we are able to determine the relative distances from the Sun to its planets orbiting around it. Looking at the parallax shift of Venus on the Sun’s surface from two different points on the Earth, we can use trigonometry to determine the distance from the Sun to Earth, and the distance from Venus to the Earth.
Figure 5: Angles during half moon
In Figure 7, the circle on the left is the Earth, the middle small circle is Venus, and the circle on the right is the Sun. Observer A and B stands at point A and point B on the diagram. Looking from point A, Venus would land on position A’ on the sun, while from point B, Venus would land on B’ on the sun. From two different points on Earth, A and B, Venus is observed as it moves across the sun .
Figure 6
Figure 7
Figure 8: Venus Transit
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