Light-Year Calculator

A light-year (ly) is the distance light travels through vacuum in one Julian year (365.25 days). Using the exact speed of light (299,792,458 m/s) and the Julian year (31,557,600 s): 1 ly = 299,792,458 * 31,557,600 = 9.4607304725808e15 m exactly. A light-year is a unit of distance, not time. The nearest star (Proxima Centauri) is 4.24 light-years away.

These are fractional light-year units. 1 light-second = 299,792,458 m (about 300,000 km). 1 light-minute = 17,987,547 km. 1 light-hour = 1,079,252,848 km. 1 light-day = 25,902,068,000 km. The Sun is 8.317 light-minutes from Earth. The Moon is 1.28 light-seconds from Earth. These units are useful for describing distances within the Solar System.

The observable universe extends approximately 46.5 billion light-years in radius. This is larger than the age of the universe (13.8 billion years) because the universe has been expanding while light was traveling to us. The most distant galaxies observed have redshift z greater than 13, corresponding to distances (comoving) of about 32 billion light-years. The cosmic microwave background was emitted about 13.8 billion years ago.

1 parsec = 3.261563777 light-years. 1 light-year = 0.30660 parsecs. Parsecs are the preferred unit in professional astronomy because they relate directly to parallax measurements. Light-years are more commonly used in popular science and educational contexts because they convey the time delay inherent in all astronomical observations: seeing a star 100 ly away means seeing it as it was 100 years ago.

1 light-year = 9.4607304725808e15 m = 9,460,730,472,580.8 km (about 9.46 trillion km). Multiply the number of light-years by 9,460,730,472,580.8 to get kilometers. For example, 4.24 light-years (Proxima Centauri) = 4.24 * 9,460,730,472,580.8 = approximately 40.1 trillion km.