In the calculation of parsecs, why do we use an angle of (90- arcsecond) degrees when ( 90 - microarcsecond) degrees are smaller? Is it out of convenience, since they are so small?

Question

anonymous · Answer

The angles, being very small, are often measured in arcseconds. Astronomers use arcseconds because one arcsecond is very close to the parallax values for stars nearest the earth, parallax in this case being the apparent angular difference in the position of a star viewed from two different places on the earth. Read more: How to Calculate Arcseconds | eHow.com http://www.ehow.com/how_8203090_calculate-arcseconds.html#ixzz21Qa5JuSg

anonymous · Answer

Arcseconds are indeed small but microarcseconds are much smaller, and my question is why don't we use microarcseconds which are much smaller?