What provides two different vantage points to observe the parallax of a distant star such as Cygnus X-1?

What provides two different vantage points to observe the parallax of a distant star such as Cygnus X-1?

• A. Earth's rotation
• B. Earth's motion
• C. The Moon's orbit
• D. Solar activity

Answer: (B)

Observing parallax relies on having two viewpoints separated in space. The Earth’s motion around the Sun provides exactly that: as Earth travels to opposite sides of its orbit over about six months, we observe the star from two very different locations, about 2 astronomical units apart. Against the far background of stars, the nearby star appears to shift slightly, and that tiny apparent motion—the parallax angle—allows us to infer distance. The Moon’s orbit would only give a tiny baseline, far too small to measure for distant stars. The Earth's rotation also changes our viewpoint but over a much smaller baseline and isn’t the method used to gauge parallax for faraway objects like Cygnus X-1. Solar activity doesn’t supply different observing positions.

Observing parallax relies on having two viewpoints separated in space. The Earth’s motion around the Sun provides exactly that: as Earth travels to opposite sides of its orbit over about six months, we observe the star from two very different locations, about 2 astronomical units apart. Against the far background of stars, the nearby star appears to shift slightly, and that tiny apparent motion—the parallax angle—allows us to infer distance.

The Moon’s orbit would only give a tiny baseline, far too small to measure for distant stars. The Earth's rotation also changes our viewpoint but over a much smaller baseline and isn’t the method used to gauge parallax for faraway objects like Cygnus X-1. Solar activity doesn’t supply different observing positions.