Light Year Calculator

Parsecs arise naturally from the parallax method of stellar distance measurement and make trigonometric calculations cleaner. The relation d(pc) = 1/p(arcsec) is simpler than converting to light-years. Professional astronomical literature, catalogs, and databases almost universally use parsecs (pc, kpc, Mpc, Gpc).

The speed of light in a vacuum is exactly 299,792,458 meters per second (approximately 300,000 km/s or 186,000 miles per second). It is a fundamental physical constant, denoted c, and appears in Einstein's famous equation E = mc². Nothing with mass can reach or exceed this speed.

Yes. Because light has a finite speed, we always see objects as they were when the light left them. The Sun appears as it was 8.3 minutes ago. Alpha Centauri as it was 4.24 years ago. The Andromeda Galaxy as it was 2.5 million years ago. When astronomers observe galaxies at redshift z=1, they see the universe as it was about 7.7 billion years ago.

As of 2026, some of the most distant objects observed are galaxies at redshifts above 10, detected by the James Webb Space Telescope. These galaxies are seen as they were less than 500 million years after the Big Bang. Due to the expansion of the universe, their current distance from us (comoving distance) is over 30 billion light-years, even though the universe is only about 13.8 billion years old.

Proper distance is the instantaneous distance at a given time, accounting for the expansion of the universe. Comoving distance factors out the expansion so that positions of objects that move only with the Hubble flow stay constant. The light-travel distance is how far light has traveled since emission. For nearby objects these distinctions are negligible, but for cosmological distances they differ significantly.

The Milky Way's disk is approximately 100,000 light-years (30 kiloparsecs) in diameter, though recent estimates suggest the disk may extend to 120,000 light-years or more in some directions. The stellar halo extends much further — perhaps 200,000 light-years. The Sun is located about 27,000 light-years from the Galactic center.

Current spacecraft travel at about 0.00002% of the speed of light (Voyager 1 at about 17 km/s). At this speed, reaching the nearest star system (4.24 light-years) would take about 75,000 years. Reaching the speed of light is impossible for any object with mass — the energy required diverges to infinity as velocity approaches c.

These are fractional light-travel distances: 1 light-second = 299,792 km (the Moon is about 1.28 light-seconds away). 1 light-minute = 17.99 million km (about 0.12 AU). 1 light-hour = 1.08 billion km (about 7.2 AU, near Jupiter's orbit). 1 light-day = 25.9 billion km (about 173 AU, well into the Oort Cloud).

The observable universe is the spherical region around us from which light has had time to reach us since the Big Bang. Its radius is about 46.5 billion light-years (the comoving particle horizon). This is larger than 13.8 billion light-years (the age of the universe times c) because space itself has expanded during the journey. The total universe may extend far beyond the observable portion.