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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