Uranus’s rotation axis is highly tilted with respect to the plane of its orbit—by 97.77° (see photo). This means that the duration and amplitude of each season are extreme on this planet.

While we often read that Uranus’s north pole points towards the Sun for 42 years or that each season lasts 42 years, this is not accurate.

The animation on the right shows the real situation. Use the buttons underneath the black circle to control it.

We can see that seasons change every 19 to 23 years approximately: northern spring, which started in 2007, will last until 2030; northern summer will stretch from 2030 to 2050; and so on. As for the poles, they point direction towards the Sun only at solstices… well, not quite “directly,” indeed, but at some 7.77° north or south of the Sun. Seasons don’t all have the same duration, because the planet has an elliptical orbit; when it’s closer to the Sun (around perihelion), it moves faster.

How Are Seasons Determined on Other Planets?

The procedure is explained in the book More Mathematical Astronomy Morsels by Jean Meeus, p. 299–300. First, one must know the values of \(\alpha_0\) and \(\delta_0\), the equatorial coordinates of the planet’s north pole (in degeres), and \(x\), \(y\), and \(z\), the rectangular heliocentric coordinates of the planet at a given moment, measured with respect to the equator and ecliptic of 2000.0. The planet’s distance to the Sun is then calculated by \(r^2 = x^2 + y^2 + z^2\), then we determine the following three parameters: \(F = \cos\ \alpha_0\ \cos\ \delta_0\), \(G = \sin\ \alpha_0\ \cos\ \delta_0\), and \(H = \sin\ \delta_0\).

Finally, the planetocentric declination of the Sun, \(d\), is given by \(\sin\ d\ = -\frac{Fx\ +\ Gy\ +\ Hz}{r}\). The spring equinox is when this declinaison goes from a negative value to a positive one; the summer solstice is when \(d\) reaches its maximal value; the fall equinox is when \(d\) goes from being positive to being negative; and the winter solstice is when the planetocentric declination of the Sun reaches its minimal value.

Meeus also gives dates calculated by the Bureau des Longitudes using the VSOP87 planetary theory and the coordinates of the north pole of Uranus published in 1992 by the US Naval Observatory. However, these coordinates have been slightly revised since (see the Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015), and the VSOP87 theory was made obsolete by the VSOP2013 and the Jet Propulsion Laboratory’s Development Ephemeris (now at version DE438). The dates provided by Meeus have thus been revised.

Seasons of Uranus*
Direction of the rotational north pole (IAU definition) : \(\alpha_0\ =\ 257,311°\), \(\delta_0\ =\ -15,175°\)
Spring Equinox	Summer Solstice	Fall Equinox	Winter Solstice
December 6, 2007	April 9, 2030	January 31, 2050	September 29, 2069
*Calculated by EcliptiQc from JPL data. These dates may differ between sources, depending on the planetary theory being used and the way to calculate them.

Note that the dates of these seasons may differ from one source to another; however, EcliptiQc takes into account the eccentricity of Uranus’s orbit, which doesn’t seem to be the case for most other sources (including, surprisingly, NASA). Also, our knowledge of the direction of Uranus’s rotation axis has been refined since the determination of some dates.

Note: In 1970, the International Astronomical Union has decided that “The rotational pole of a planet or satellite which lies on the north side of the invariable plane shall be called north, and norhern latitudes shall be designated as positive.” This means that, as seen from its north pole, Uranus rotates about its axis clockwise, as opposed to other planets (except Venus), which rotate about their axis counterclockwise. According to this definition, northern spring begun in 2007; using the older convention, which made the northern pole the one about which the planet rotates counterclockwise, would mean the northern fall begun in 2007.

© 2024 EcliptiQc · English version published January 11, 2021, by Pierre Paquette