Very long-range interferometry involves the simultaneous measurement of the times of signals from very distant astronomical objects such as ______.

• A. Planets
• B. Quasars
• C. Moons
• D. Satellites

Answer: (B)

Very long-range interferometry relies on recording and comparing the arrival times of waves from distant celestial sources observed by widely separated telescopes. To achieve the highest precision, you need a source whose position is effectively fixed on the sky, so the reference frame stays stable over time. Quasars are ideal for this: they are incredibly distant, bright, compact radio sources whose apparent positions change negligibly over human timescales, providing a steady reference point for connecting observations across vast distances. In contrast, planets, moons, and satellites are relatively nearby and move noticeably due to orbits and parallax, so they don’t serve well as fixed reference points for very long-baseline measurements. Hence, objects like quasars are used in very long-range interferometry.