The zero point changes the label
The tropical zodiac starts at the equinox. A sidereal zodiac uses a specified offset, called an ayanamsa, to relate that reference to its own zero point. Precession means the offset changes with date; it is not one fixed number that can be applied to every historical chart.
Neither system’s twelve equal signs are the irregular astronomical constellation regions. Selecting Sidereal does not turn this table into a constellation-boundary detector. A label such as Aries identifies a 30° interval in the selected zodiac, so record which zodiac supplied the label.
A named ayanamsa is part of the calculation
Sky Degrees offers Lahiri and Fagan–Bradley. Two tables can both say ‘sidereal’ and still use different reference offsets. Match the named ayanamsa as well as the date and time. Do not assume a different sign means that one table used an incorrect birth date or the wrong planet.
At finer precision, the reference frame also matters. This implementation converts an apparent tropical longitude to its traditional mean sidereal convention by removing the nutation-in-longitude term and subtracting the selected mean ayanamsa. A rounded rule such as ‘subtract 24°’ is a teaching shortcut, not an exact reproduction of the engine.
Compare a fixed sample instead of two moving timestamps
Open Daily ephemeris and choose a month, IANA time zone and daily sampling time. Record a row’s UTC instant, planet and longitude with Tropical selected. Change only the zodiac to Sidereal, select Lahiri or Fagan–Bradley, apply the settings and compare the same date. Check the UTC record again before interpreting the difference.
Calendar mode is useful for this exercise because it holds the requested sample instant fixed. Two separate Current sky requests occur at different times. Current sky and Calendar share browser preferences, so changing the zodiac in one also affects later requests in the other. Restore your preferred setting afterward if needed.
A CONCRETE EXAMPLE
Arithmetic example: a sign change without a change in the sky
Take an invented tropical longitude of 15°, which is Aries 15°. For this arithmetic exercise only, use a total reference offset of exactly 24°. Subtraction gives −9°; adding 360° wraps it to 351°, or Pisces 21°. This is not a computed Lahiri or Fagan–Bradley value for a particular date.
A second invented position at 195°, Libra 15°, becomes 171°, Virgo 21°, under the same offset. The two points remain 180° apart. A consistent rotation changes the coordinate labels while preserving their longitude separation at that instant. It does not change the physical sky or add evidence for an astrological interpretation.
Check the time and the result.
Keep the sample time, time zone and zodiac beside the result. Current sky and Calendar share your settings in this browser.
Compare a fixed ephemeris sampleCommon questions
Which zodiac gives the more accurate current position?
They answer the coordinate question using different origins. First match the astronomical model, time, observer convention and reference definition. A zodiac preference alone does not establish numerical accuracy or validate a prediction.
Does changing the time zone explain a tropical–sidereal difference?
Not if both rows refer to the same UTC instant. In Calendar, however, keeping the same local clock time while changing zones can select a different instant. Check that distinction before comparing zodiac settings.
Method and limits
The current-sky and daily ephemeris tools use Skyfield with JPL DE440. A dated sample is a historical calculation at the stated time and zodiac; it is not a current position. Worked arithmetic and geometry examples are labelled separately.
The table reports geocentric positions and sampled motion. It does not calculate local visibility, illuminated fractions or exact event times. Keep source, time and coordinate settings with each comparison; the result’s Calculation basis provides the full record.
- Sky Degrees — chart settings, zodiac and ayanamsa
- Skyfield — positions, reference frames and coordinates