Stellar parallax is the apparent shift in position of nearby stars as the Earth moves around the Sun. Which of the following best explains this shift?

• A. The Earth's orbit around the Sun.
• B. The tilt of the Earth's axis relative to its orbit.
• C. The Earth's rotation on its axis.
• D. The Moon's orbit around Earth.

Answer: (C)

Stellar parallax comes from a change in our viewpoint as the Earth travels around the Sun. Over six months, Earth moves from one side of the Sun to the other, giving a baseline of about 2 astronomical units. When you compare a nearby star to faraway background stars from these two vantage points, the nearby star appears to shift slightly against the distant stars. The amount of this apparent shift—the parallax angle—is tiny and decreases with distance, so only relatively nearby stars show a measurable parallax (for a star at 1 parsec, the parallax is 1 arcsecond).

The rotation of the Earth on its axis isn’t what creates this annual baseline; daily rotation simply changes your local vantage a few thousand kilometers on the planet, which is negligible for stars that are light-years away. The tilt of the Earth's axis governs seasons and orientation, not the baseline that causes parallax. The Moon’s orbit around Earth is far too small to produce a measurable stellar parallax.

So the best explanation is that the apparent shift arises from Earth’s orbit around the Sun, providing the changing viewpoint that makes nearby stars appear to move against distant background stars.