Right Ascension (RA, α) is the celestial equivalent of longitude on Earth, measuring how far east an object appears along the celestial equator from a starting point called the vernal equinox. Think of it as cosmic longitude that helps astronomers pinpoint exactly where to find stars, planets, and galaxies in the night sky.</p><p>RA is measured in hours, minutes, and seconds rather than degrees, with a full 360° circle divided into 24 hours (15° per hour). For example, the bright star Sirius has an RA of about 6h 45m, while the Andromeda Galaxy sits at roughly 0h 43m. This system means that as Earth rotates, objects with the same RA will cross your local meridian (highest point in the sky) exactly 4 minutes earlier each night.</p><p>The vernal equinox serves as RA's "prime meridian"—the zero point where the Sun crosses the celestial equator each March. This coordinate system was refined in the 18th century as telescopes improved, allowing astronomers to create precise star catalogs. Today, RA coordinates are essential for telescope automation, space missions, and astronomical databases. When paired with declination (celestial latitude), RA provides a complete address system for the cosmos, enabling astronomers worldwide to locate and study the same celestial objects with pinpoint accuracy.