April 24

April 24

Hubble's Launch Day

Sun Position

The Sun is in Taurus near +22° declination. Northern hemisphere days are long, with astronomical twilight lasting well past 9 PM at mid-latitudes; southern hemisphere nights are long and excellent for observing.

Sky Highlight

April 24, 1990, was the launch date of the Hubble Space Telescope. Hubble's observations of Cepheid variable stars in distant galaxies in the 1990s pinned down the Hubble constant with far greater precision than was previously possible, resolving a decades-long disagreement about the universe's expansion rate. M100 in the Virgo Cluster, about 55 million light-years away, was one of the key galaxies used in that campaign.

Deep Sky Object

M100 (NGC 4321), grand design spiral galaxy, Virgo Cluster, about 55 million light-years. M100 is one of the brightest face-on spirals in the Virgo Cluster, with clearly defined spiral arms in large amateur telescopes; it was one of the first galaxies in which Hubble Space Telescope resolved individual Cepheid variable stars, providing a key rung in the cosmic distance ladder. Best from northern mid-latitudes.

Featured Star

Kochab (β UMi) is an orange K4III giant 130.9 light-years away, the second-brightest star in Ursa Minor. Like Thuban, its history is largely precessional: it served as the north pole star from roughly 1500 BCE to 500 CE, used by Greek and Roman navigators, and is now 16° from the celestial pole but still circumpolar from all northern mid-latitudes.

Around This Date

  • April 24, 1990NASA's Hubble Space Telescope launched aboard Space Shuttle Discovery on mission STS-31, beginning more than three decades of transformative astronomical observation.
  • April 25, 1990Hubble Space Telescope was deployed from Discovery's cargo bay and began checkout operations, with early images later revealing a spherical aberration in the primary mirror that required a 1993 servicing mission to correct.

Kochab held the pole for two thousand years and then the precession moved on; the sky keeps rearranging its landmarks whether we notice or not.