April 18
The Triple Blue
Sun Position
The Sun is in Aries near +19°, approaching the Aries-Taurus border. Northern hemisphere evenings are long; southern hemisphere autumn nights are well established.
Sky Highlight
Alpha Centauri (Rigil Kentaurus), though not well placed from northern latitudes, becomes a useful southern-sky landmark in April evenings for observers in the southern hemisphere. It reaches a reasonable evening elevation in April from mid-southern latitudes, and its proximity (4.37 light-years) makes it the closest stellar system with naked-eye brightness.
Deep Sky Object
NGC 5128, Centaurus A, radio galaxy, roughly 13 million light-years. Centaurus A is the nearest radio galaxy and one of the most studied objects in extragalactic astronomy, hosting a massive black hole and an active jet visible across the electromagnetic spectrum; its distinctive dark dust lane crossing the elliptical host is visible in small telescopes. Southern hemisphere object; very low or below the horizon from latitudes north of about 20°N.
Featured Star
Hadar (β Cen) again: 390 light-years away, a blue B1III giant that is actually a triple system, with the close inner pair so tight that even large telescopes cannot split them visually. It is the 11th brightest star in the sky and a southern sky fixture, pointing along with Alpha Centauri toward the Southern Cross.
Around This Date
- April 18, 1955Albert Einstein died in Princeton, New Jersey; his general theory of relativity remains the foundational framework for understanding black holes, gravitational waves, and the expansion of the universe.
- April 20, 1972Apollo 16 landed on the lunar surface at the Descartes Highlands, with John Young and Charles Duke becoming the ninth and tenth humans to walk on the Moon.
Hadar looks like a single star and is actually three; the Southern Cross pointer does its job regardless.