Exercise:

On April 1st, what is the Sun's approximate ecliptic longitude?

At the spring equinox, on or around March 21st,
the Sun's ecliptic longitude is exactly 0°.
11 days have elapsed since then,
so the Sun's longitude will be approximately 11°.
(Only approximately,
because the ellipticity of the Earth's orbit means that the Sun does not move around the ecliptic at a steady speed.)

And approximately what is Greenwich Sidereal Time at midnight on April 1st?

GST and GMT are the same at the autumn equinox,
so they are exactly 12 hours out of phase at the spring equinox.
So GST is 12h at midnight on March 21st, approximately.

Eleven days later, on April 1st,
GST will have got ahead of GMT by 11 x 3.94 minutes = 43.3 minutes.
So GST at midnight on April 1st will be approximately 12h43m.

Back to "the Sun's motion".