$A$ tachometer is a device used to measure:

When the planet Jupiter is at a distance of $824.7$ million kilometers from the Earth,its angular diameter is measured to be $35.72''$ of arc. Calculate the diameter of Jupiter.

What is the uncertainty in time measured by a cesium atomic clock?

In an experiment, a set of readings are obtained: $1.24 \ mm, 1.25 \ mm, 1.23 \ mm, 1.21 \ mm$. The expected least count of the instrument used in recording these readings is . . . . . . $mm$.

Define astronomical unit,light year,and parsec.

The principle of 'parallax' is used in the determination of distances of very distant stars. The baseline $AB$ is the line joining the Earth's two locations six months apart in its orbit around the Sun. That is,the baseline is about the diameter of the Earth's orbit $\approx 3 \times 10^{11} \; m$. However,even the nearest stars are so distant that with such a long baseline,they show parallax only of the order of $1''$ (second of arc) or so. $A$ parsec is a convenient unit of length on the astronomical scale. It is the distance of an object that will show a parallax of $1''$ (second of arc) from opposite ends of a baseline equal to the distance from the Earth to the Sun. How much is a parsec in terms of metres?