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Using an earth curvature calculator, I found that at a distance of 4 km (the length of LIGO's arms), more 1.26 m is hidden by the horizon. When constructing LIGO, did they account for the curvature of the Earth to allow the beam to travel in a straight line?

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    $\begingroup$ Figure it this way: the suspended mirrors at the ends of the arms are (IIRC) about 40 cm in diameter. If they hadn't considered the matter the machine wouldn't work. $\endgroup$ – dmckee Feb 12 '16 at 4:21
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    $\begingroup$ Indeed. The 4 feet of difference over 4 kilometers isn't exactly an engineering challenge compared with building the tunnel in the first place. $\endgroup$ – Jon Custer Feb 12 '16 at 5:13
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The answer is yes, the earth's curvature was factored into the LIGO design. Read the LIGO wbpages, here,

LIGO’s arms are long enough that the curvature of the Earth itself was a complicating factor when installing the vacuum tubes. It wasn’t enough for LIGO’s civil engineers to smooth a level path and assemble each arm’s tubes in a straight line. To ensure a perfectly flat, straight, and level beam path, the Earth’s curvature (more than a vertical meter over each 4 km arm) was countered by GPS-assisted earth-moving and high-precision concrete work.

and here

Curvature of the Earth: LIGO’s arms are so long that the curvature of the Earth is a measurable 1 meter (vertical) over the 4 km length of each arm. The most precise concrete pouring and leveling imaginable was required to counteract this curvature and ensure that LIGO’s vacuum chambers were truly "flat" and level. Without this work, LIGO's lasers would hit the end of each arm 1 m above the mirrors it is supposed to bounce off of!

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