Scientists know exactly how long it takes Earth to rotate because they have been making that measurement for decades using an extremely precise technique called
Very Long Baseline Interferometry (VLBI).
By observing at the very short wavelength of 7 millimeters, and by combining the signals from a world-spanning array of telescopes through the technique of
very long baseline interferometry (VLBI), the astronomers were able to produce a radio image of M87's core with a resolution of 120 millionths of an arcsecond in the east-west direction -- nearly 1,000 times sharper than a typical Hubble Space Telescope view.
Very long baseline interferometry (VLBI) is the process of using several or many telescopes that are distant from one another, carefully combining their signals to make them effectively act as one large telescope, peering intently at a tiny portion of the sky.
Very long baseline interferometry measurements of the distance to the galaxy M106 (NGC 4258) favor a "short" distance scale for cosmological purposes, not a "long" scale (December issue, page 44).
That launch vehicle, scheduled for completion in early 1995, may find its first use in a
Very Long Baseline Interferometry experiment planned for later that year.
Another technique,
Very Long Baseline Interferometry (VLBI), acts as a kind of GPS for Earth.
Very long baseline interferometry is paradoxically successful.
They confirmed this information with
very long baseline interferometry, in which a number of radiotelescopes on different continents observe a quasar at the same time and compare the signals to get information about the relative motion of the observatories.