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Astronomical Unit Converter

Convert between astronomical distance units: AU, light years, parsecs, and km.

Conversions

km1.49598e+8
AU (Astronomical Units)1
light-seconds499.005
light-minutes8.31675
light-hours0.138612
light-days0.00577552
light-years1.5813e-5
parsecs4.8481e-6

Reference Distances

Earth–Sun distance1 AU
Light from Sun to Earth~8.3 min
Nearest star (Proxima Cen.)4.24 ly
Milky Way diameter~100,000 ly
Andromeda Galaxy~2.537 million ly

About this tool

The Astronomical Unit Converter helps you translate distances across the cosmos into familiar units. When astronomers and space enthusiasts talk about how far stars and galaxies are from Earth, they use specialized measurements—the astronomical unit (AU) for nearby objects, light years for distant stars, parsecs for professional measurements, and kilometers for precision calculations. This tool instantly converts between these four fundamental cosmic distance scales, making it easy to understand the true scale of the universe.

To use the converter, simply select your starting unit, enter a number in the input field, and the tool automatically displays the equivalent values in all other units. Whether you're reading about exoplanets that orbit 5 AU from their star, learning that Proxima Centauri is 4.24 light years away, or working with astronomical data measured in parsecs, this converter lets you visualize distances in whichever unit makes the most sense to you. It's equally useful for homework, research, or satisfying curiosity about space exploration and cosmology.

One practical tip: the AU is most useful for distances within our solar system, light years resonate with popular science discussions, and parsecs are the standard in professional astronomy. The tool handles very large numbers with full precision, so you can confidently convert even the distances to the most remote galaxies observed by modern telescopes.

Frequently Asked Questions

Code Implementation

# Astronomical unit conversions (all in km)
TO_KM = {
    "km":  1,
    "au":  149_597_870.7,
    "ls":  299_792.458,        # light-second
    "lm":  17_987_547.48,      # light-minute
    "lh":  1_079_252_848.8,    # light-hour
    "ld":  25_902_068_371.2,   # light-day
    "ly":  9_460_730_472_580.8, # light-year
    "pc":  30_856_775_814_913.67, # parsec
}

def convert(value, from_unit, to_unit):
    km = value * TO_KM[from_unit]
    return km / TO_KM[to_unit]

# Examples
print(f"1 AU = {convert(1, 'au', 'km'):,.1f} km")
print(f"1 AU = {convert(1, 'au', 'ls'):.2f} light-seconds")
print(f"1 AU = {convert(1, 'au', 'lm'):.2f} light-minutes")
print(f"Nearest star (4.24 ly) = {convert(4.24, 'ly', 'au'):,.0f} AU")
print(f"1 parsec = {convert(1, 'pc', 'ly'):.2f} light-years")

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