How is the sun's equatorial circumference measured?

Question

How is the sun's equatorial circumference measured by scientists?

Answer 1

It is quite simple to get a reasonable measure of the sun's diameter, using a pinhole, a screen and the formula for experiment 2 on this At Home Astronomy article: "Finding the size of the sun and moon".

From the 'At Home Astronomy' article, the sun's diameter can be calculated as:

$$\frac{\text{Diameter of the sun's image}}{\text{Distance from the pinhole to the paper}}\times\text{Earth-sun distance} = \text{Diameter of the sun}$$

Edited to add:

The first recorded attempts to determine the distances and sizes of the Earth and Moon were performed by Aristarchus (310 - 230 B.C.), these were not very accurate (this information is also in the first link I gave originally). Once the distance was known (or estimated), a measure for the size could be determined.

His method was to determine the angle to the sun when the moon was half full, hence at a right angle to the observer (a potential source of error) and measure the angle to the sun (as in the diagram below)

(Source)

I hope this clears up the inadequacies of my original post.

Answer 2

It should be simply as below: $C = \pi d$