What is the difference between Sidereal Time and Solar Time?

Sidereal and Solar time are two methods of measuring the three distinct movements of 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.

Sidereal time measures time based on the Earth’s rotation relative to a distant star:

• A Sidereal day is the time it takes for the Earth to complete one full rotation relative to the star. (23 hr 56’ 04”).

• A Sidereal year is the time it takes the Earth to complete one complete orbit  (360°) around the Sun, again relative to a distant star. (366.256 days)

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.

• 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 that is why a Solar day 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.

• Having a leap year every four years allows us to synchronize our calendar to the orbit of the Sun.

The rotation of the axis:

As the Earth rotates, it introduces a wobble onto its axis. Similar to the spinning of a top, the Earth’s axis describes a slight arc. By extending this arc onto the celestial equator from the North Pole, it eventually completes a 360° rotation. This takes approximately 25,600.

• 71 years to move 1°, = 26,000 (Sidereal days)

• 26,000 days x 360° = 9,360,000 days (to complete one full rotation)

• 9,360,000/366.256 = 25,556 years (25,600)

This phenomenon is referred as the Precession of the Equinox. The axis spends about 2000 years in each sign, defining the Astrological Ages. We are currently transitioning from the Age of Pisces to the Aquarian Age.

How do Astrologers Reconcile Solar Time with Sidereal Time?

Sidereal time not associated with any calendar here on Earth. You will see in the American Ephemeris a Sidereal time listed that corresponds to the 0hr of every day of our calendar. The planetary positions are also listed for the day and 0hr. Due to the rapid movement of the Moon two positions are listed, 12 hrs apart. Having both Sidereal Time and the corresponding calendar day and time allows us to synchronize the two measuring systems once a day.

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°).

Sidereal time resets to 0 when the Vernal Equinox, the point when the Sun crosses the celestial equator and also crosses the observer's meridian, an imaginary arc that goes through both poles and the observer’s zenith.

This is a universally accepted convention, much like how our calendar year begins on Jan 1st.