[LEAPSECS] Worlds apart

[Rob Seaman]

To return to a message from several days ago, Steve Allen wrote:

> If I ask the question "Is Pluto a planet?"  I know where I can find
> the authoritative answer.  The proceedings from the IAU GA in 2006
> make it clear that the answer is "no".

When Pluto was demoted, the asteroid Ceres was promoted to dwarf planet.  Ceres contains a third of the mass of the asteroid belt, and indeed for the first half century after it was discovered on January 1, 1801 it was considered a planet.*  At issue is not just "What is Pluto?"  At issue is the definition of the word planet.  These are not just musings about hypothetical places.  The Dawn spacecraft will reach Ceres in the spring of 2015.

> If I ask the question "Is Universal Time a measure of earth rotation?"
> I can find the authoritative answer in the proceedings from many
> different IAU General Assemblies over an interval of more than half a
> century.  The answer is "yes", and the same answer is present in
> textbooks stretching over several centuries, and Louis Essen (inventor
> of the cesium chronometer) agreed.

Indeed: “…time scales have always served the three quite distinct functions, of giving the time of day, the season of the year, and also a measure of time interval or duration. Any new scale must continue to serve these purposes, if it is to be of universal use and although an atomic clock can provide a very precise scale by simply counting and recording the number of seconds that have elapsed since some arbitrary zero, the time of day and the season of the year can be obtained only by astronomical measurements.”  (Essen, L., 1968, “Time Scales.” Metrologia, Vol. 4, p. 161.)

Even more fundamentally, by ceasing leap seconds the ITU seeks to silently redefine the word day.  But this is not subject to debate.  Referring to a day means referring to a synodic day, that is, a day by mean solar time.  This is true on Pluto and Ceres and it is true on Earth.  It is not so much a definition as an observation about nature.  There are one fewer days per year than sidereal rotations (due to lapping the Sun) for planets, dwarf planets, asteroids and moons throughout this solar system and others.  (http://futureofutc.org/preprints/files/28_AAS_13-515_Seaman.pdf)

Venus is a retrograde rotator so the period is negative but the same minus-one formula applies.  There are -0.92 sidereal rotations per Venus year, but -1.92 days per year on Venus.  Venus completes one rotation relative to the stars every 243 Earth days.  But the length of the day on Venus is a dramatically different value of 116.75 Earth days.  The two concepts are distinct.  The meaning of the word “day” is not a free parameter.  Venus is often considered Earth’s twin, but on Venus we wouldn’t be having this discussion (even for the few seconds before our flesh melted).

At the same time that Dawn reaches Ceres another spacecraft called New Horizons will be approaching Pluto.  When New Horizons was launched in 2005, Pluto was still considered a planet.  That the definition changed in flight does not impact the scientific value of the mission.  Whatever they are called, Pluto remains a member of the Kuiper Belt and Ceres of the main asteroid belt.  Neither the ITU nor the IAU can change those facts...or the definition of the day.  Their actions should aspire to agree with physical reality.

Rob Seaman
National Optical Astronomy Observatory
—

* As the element plutonium was named after the recently discovered planet Pluto, the element cerium was named for Ceres; the metal was isolated just two years after the planetoid was discovered by Giuseppe Piazzi.  Other elements with astronomical names are mercury, uranium, neptunium, tellurium, selenium - and palladium for the second asteroid discovered, Pallas (by H. Olbers about a year after Ceres).  Helium was first discovered not on Earth, but in the Sun’s atmosphere.  We are residents not only of the Earth, but of our solar system.

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