Venus is covered in a thick layer of clouds, one reason that it appears so bright in the sky. Ancient astronomers had a good idea of what (since Copernicus) we know as its orbital period; the modern measurement is that Venus takes 224.65 days to complete one revolution around the Sun, a Venusian year. Because of the clouds, however, it has been difficult to measure the length of the Venusian day since the nominal method of watching a visible surface feature rotate around 360 degrees is not possible. In 1963, Earth-based radar observations penetrated the cloud cover and were able to measure a rotation rate of 243 days; more surprising is that Venus rotates on its axis in the opposite direction from that of most planets, so-called retrograde rotation. Subsequent ground-based radar studies came up with inconsistent values for the length, differing by about six minutes. The Magellan spacecraft completed its 487 day orbital mapping program in 1991 and concluded the correct number was slightly different still: 243.0185 days with an uncertainty of about nine seconds. But subsequent missions and ground-based observations found that the rate of rotation was actually not constant but seemed to vary, with models arguing that solar tidal torques and atmosphere drag on the surface could account for at least some of the variation.
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Source: Phys.org