51�Թ�

Naval Observatory routinely uses 34 minutes of arc for the angle of refraction and 16 minutes of arc for the semidiameter of the sun's disc.

This is because the half of the equatorial diameter enormously outmeasures even the greatest apparent semidiameters of the sun and of the moon, even taking as a norm the smallest horizontal parallax of the moon.

The second is by measuring the semidiameter of the Earth: For as the circumference makes knowne the diameter, so doth this the circumference.

Suppose, for example, that looking at the moon I should say it were fifty or sixty semidiameters of the earth distant from me.