HOW BIG IS THE SUN? A MEASUREMENT FROM INDIA’S ADITYA-L1 SPACECRAFT

Abstract

We present a step-by-step guide to measuring the Sun’s apparent angular radius from real spacecraft images, using Level-1 full-disk data from the Solar Ultraviolet Imaging Telescope (SUIT) onboard India’s Aditya-L1 mission. A brightness threshold isolates the solar disk, and an algebraic circle fit to the disk boundary recovers its pixel radius. Three images taken on consecutive days in May 2025 give an angular radius of 936:4 0:300, corresponding to a Sun-observer distance of 1:532 108 km (1:024AU), and a physical solar radius of (680;900 200) km, approximately 2:1% below the accepted value of 695;700 km. The small shortfall is a predictable and well-understood effect of brightness-based limb detection. A comparison with published heliometric, helioseismic, and space-photometric measurements confirms that precision determinations consistently cluster near the IAU 2015 nominal value, and that our 2:1% deviation is fully accounted for by the UV-threshold limb definition used here. A self-contained Python script is provided so that every figure can be reproduced on a standard laptop.