An astronomer would most likely use parallax to

measure the distance to the stars

Explanation:

Parallax is used in astronomy to measure the distance to the stars.

The parallax method uses the change of position of the earth due to its orbit around the sun.

When the earth moves in its orbit, the nearest stars seem to move with respect to the farthest stars, using parallax we can calculate the distance to this stars.

Measurements are made at two different points of the orbit (usually the measurements are separated by six months, so that the earth is in opposite places of its orbit) with respect to the position of the star from which astronomers want to know the distance and the stars farther away. With this data, and the use geometry is possible to calculate the distance to the given star.

6 cos 34, 6 sin 34 trig