Here's why:
* Parallax: Stellar parallax is the apparent shift in a star's position against the background of distant stars as observed from two different points in space.
* Baseline: To measure this shift, we need a known distance between the two observation points. The Earth's orbit provides this baseline.
* Method: Astronomers measure the position of a star at two different points in Earth's orbit, typically six months apart when the Earth is on opposite sides of the Sun. The larger the angle of the apparent shift (parallax angle), the closer the star.
Think of it like this: Imagine holding a finger in front of your face and closing one eye, then the other. Your finger appears to shift against the background. The further your finger is, the smaller the shift. Similarly, stars closer to us show a greater parallax than distant stars.
The baseline of Earth's orbit is about 186 million miles (300 million kilometers) in diameter, which is essential for measuring the distances to stars, particularly those within a few hundred light-years.
