I have started teaching a Math course in Astrology and while most of my students accept at face value the steps to construct a chart without a computer, others really challenge me. My latest student really struggled with why Sidereal time had to be reconciled with Solar time in order to define the angles of the chart and the inner house cusps. I really struggled to come up with an explanation that was accessible to an audience that may not have a strong mathematical back ground. After about two weeks of driving myself insane, I think I’ve come up with something that explains the differences between Sidereal Time and Solar Time. This understanding can be the starting point to what is not a difficult process (creating the chart) but it is a time consuming process with many, many steps. Here we go:

There are three distinct movements associated with the Earth.The daily rotation of the earth on its axis, the Earth’s orbit around the Sun, and the rotation of the axis itself. These movements are measured by Sidereal and Solar time.
Solar time is relative to the Sun, while Sidereal Time is measured by calculating the the Earth’s movements relative to a distant star.

What is Sidereal Time?
Sidereal time is a way of measuring time based on the Earth’s rotation relative to a distant star, rather than the Sun. 
A Sidereal day is the time it takes for the Earth to complete one full rotation relative to the stars. (23 hr 56’ 04”).
A Sidereal year is the time it takes the Earth to complete one orbit (360°) around the Sun, again, relative to the stars.(366.256 days).

What is Solar Time
Solar time is a way of measuring time based on the Earth’s rotation relative to the Sun.
A Solar day is the time it takes to complete one rotation on its axis, relative to the Sun.
The earth must rotate slightly more than 360° to return to the same spot because the Earth is orbiting at the same time it is rotating, and is 24 hr 00” 00” in length.
A Solar year is the time it takes to complete one orbit around the Sun, 365.25 days.
This is why we have a leap year every 4 years.

Why does the Ephemeris “reset” sidereal time to 0 every September? 

The daily notations of Sidereal Time in the American Ephemeris increase by slightly less than 4 minutes each day. This is measuring how much of the orbit around the Sun the Earth completes each day (4’ = 1°), not the rotation of the Earth on its axis. (23hr56’04”)

In the American Ephemeris, Sidereal time resets to 0 when the Vernal Equinox, (the point when the Sun crosses the celestial equator) also crosses the observer’s meridian. This is a universally accepted convention, much like how our calendar year begins on Jan 1st. 

The Precession of the Equinox (the rotation of the axis itself)

In the context of the Precession of the Equinox, this September to September period is also referred to as a Sidereal Day. It is a measurement of the rotation of the Earth’s axis, caused by the rotation of the Earth. Similar to the spinning a top, the axis itself describes a slight arc. By extending this arc onto the celestial equator from the North Pole, the arc eventually completes one 360 degree rotation. This takes approximately approximately 25,600.

• 71 years to move 1°, = 26,000 (23 56′ 04″ Sidereal days)
• 26,000 days x 360° = 9,360,000 days (360° rotation of the axis)
• 9,360,000/366.256 = 25,556 Sidereal years (25,600)