Where is the eastern horizon, solstices and equinoxes in relation to one another?
Revolving-about-Sun Earth (E)-to-Sun (S) direction ES is the eastern horizon. Equinoxes are the instants when the Earth's poles are in the ecliptic. At solstices, poles are farthest from the ecliptic.
For the same observer, the morning East is seen as evening west, due to the Earth's day-night spin about its axis.
An equinox is the instant ( by about 21st March and 23rd September ) at which Sun is right overhead, at the location where the equator transits the ecliptic.
When this transit is farthest away from the ecliptic, it is a solstice ( by about 21st June and 21st December).