Vol. Science · Astronomy · The equinoxes

Precession
the slow cone

Earth’s axis leans 23.44° from straight up-and-down to its orbit. NASA’s Earth Fact Sheet lists that obliquity. Over one year the lean stays aimed the same way — that is the seasons desk. Over a much longer time the aim walks a cone. The north celestial pole, the point the axis aims at among the stars, drifts. NASA StarChild calls the motion precession and rounds one full cone to about 26,000 years. An archived NASA GSFC lesson uses the same classroom round number: about one full circle in 26,000 years. Today the axis points near Polaris. StarChild names other stars for other ages: Thuban in the year 3000 B.C., and Vega in about 13,000 years. The same drift is why the fact sheet’s tropical orbit period, 365.242 days, is shorter than its sidereal orbit period, 365.256 days. Seasons follow the equinox, not a fixed star. A shorter spin against the stars is the sidereal day. A longer phase cycle is the synodic month.

Classroom schematic — a face-on circle of the north celestial pole, and a small Earth whose tilt is held at the fact sheet’s 23.44°. The circle’s radius is that obliquity: the pole stays about 23.44° from the ecliptic pole. The GSFC lesson rounds the same radius to about 23.5°. The drawing is not a star chart. Polaris, Thuban, and Vega are signposts at cited times, spaced along a steady 26,000-year loop. The gap that puts Polaris just off the marker is exaggerated so the two can be told apart. StarChild says the axis points almost exactly at Polaris. The archived lesson says within about one degree. This page does not pick a sharper separation. The fact sheet’s North Pole of Rotation formulas — right ascension 0.00 − 0.641T, declination 90.00 − 0.557T, with T in Julian centuries from 12:00 UT 1 Jan 2000 — are printed below as the near-term drift. They are not run out around this circle. Not an ephemeris. Not a pole finder. Not for pointing a telescope. Not astrology. The zero of the slider is a schematic present near Polaris, not your computer’s clock.

Literacy · the moving equinox

Axial precession

The spin axis slowly changes the direction it points, tracing a cone. StarChild: Earth bulges at the equator, and the gravitational attraction of the Moon and Sun on that bulge is the nudge. A spinning top does the same thing on a much faster clock. The archived GSFC lesson says the same cause — the equatorial bulge, and the attraction of the Moon and Sun — and that the axis of the cone is perpendicular to the ecliptic. One full walk is about 26,000 years on both pages. Hipparchus, around 130 B.C., is who StarChild credits with the first estimate. The GSFC lesson adds his comparison: in the preceding 169 years the equinox intersections had moved by 2 degrees. This slider does not turn 2 degrees in 169 years into a period. It uses the lessons’ round 26,000.

Tropical and sidereal years

The Earth Fact Sheet lists a tropical orbit period of 365.242 days and a sidereal orbit period of 365.256 days. The tropical year brings the equinox back. The sidereal year brings the Sun back to the same stars. The tropical number is the shorter one. The equinox itself is moving, so the Sun meets that moving point before a full circuit against the stars is done. At the thousandths printed on the sheet, 365.256 minus 365.242 is 0.014 days. That 0.014 is the subtraction, not a third measurement, and this page does not divide the two periods into a sharper cycle than “about 26,000 years.” The sidereal day is the same kind of split on a spin: one turn against the stars is shorter than the mean solar day. The synodic month is the split the other way: the phase cycle is the longer one.

Seasons follow the equinox

A season is which hemisphere leans into the Sun, not which star is behind the Sun. Over one year the axis stays parallel. That picture is the seasons desk, and its tilt is this same obliquity. The GSFC lesson’s long picture: right now northern winter happens in the part of the orbit where the north end of the axis points away from the Sun, near the closer distance. About 13,000 years from now, in that same part of the orbit, the axis will point toward the Sun, so that closer distance falls in northern summer. The stars that share the sky with a given season therefore change. The equinoxes and solstices still define the seasons. Half of this desk’s 26,000-year classroom loop is 13,000 years, the same round number both lessons use. The “~½ cycle” stop puts those two sentences on one schematic mark. It is not a measured date.

Pole stars on this sketch

StarChild: right now the axis points almost exactly at Polaris. In the year 3000 B.C. the North Star was Thuban, also called Alpha Draconis. In about 13,000 years Vega will be the North Star. In 26,000 more years Polaris will be the pole star again. This drawing puts Polaris at the schematic present, Vega at half a classroom cycle, and Thuban about 5,000 years before that present — 3,000 plus 2,000, a classroom count from the year 3000 B.C. to a year-2000 now, with no adjustment for the missing year zero. On a steady loop that same place is also 26,000 minus 5,000, or 21,000 years ahead. The 21,000 is that subtraction. StarChild does not publish it as a date when Thuban returns. The button “Thuban label” only moves the marker to the signpost.

What the circle leaves out

The fact sheet’s pole formulas are a short drift: as T increases, both printed terms decrease. They are not a 26,000-year ephemeris, and this page does not evaluate them into a right ascension and declination for a year on the slider. The GSFC lesson says the inclination of the axis to the ecliptic also changes. It does not print a second obliquity for this desk to animate, so the tilt stays 23.44°. The archived page also prints a closest approach of the pole to Polaris “in 2017,” and the page itself says it is no longer being updated and may be outdated. This desk does not use that year. StarChild does not name Gamma Cephei. Neither does the GSFC lesson’s main text as opened for this page. The sketch therefore has no Gamma Cephei stop and no invented date for one. Other stars do take a turn. This drawing only labels the ones those two pages name. It is not a finder chart for local sky, and it is not the orbit-as-ruler on the parallax desk.

Sources · public NASA education

Classroom picture of a slow cone. Not an ephemeris, not a pole finder, not for pointing a telescope, and not astrology. Years on the slider are counted from a schematic present near Polaris, on a steady loop of the lessons’ about 26,000 years. Star positions are those cited times placed on that loop, not catalog angles. The 5,000-year Thuban offset is 3,000 plus 2,000. The 21,000-year signpost is 26,000 minus that 5,000. Neither offset is a separate NASA measurement. Earthrise preview: NASA / Bill Anders (public domain).

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