Lunar Grazing Occultations
What are they?
Why are they Valuable?
Are they fun?
What is a Lunar Occultation?
• An “occultation” happens when one celestial object
moves in front of a much smaller more distant celestial
object
• (If the objects are comparable in size, it’s usually called
an “eclipse” instead)
• As the moon moves through it’s orbit, it passes in front of
distant stars or planets. This essentially casts a shadow
on the Earth and from within this shadow, you can watch
the star approach the moon and wink off when it
disappears, and / or wink on when it reappears
• Here are some bright star lunar occultation shadow
paths in 2014 visible from Santa Cruz…
Some Very Cool
Occultation/Graze Videos
• A waning crescent moon graze of the 1st
magnitude star Spica (YouTube 4min 21sec)
• Waxing cresent graze of 2 Scorpii. 22 events,
clear double star with step events. (Dave Gault)
(YouTube 4:40). Note Kiwi time and frame
stamping.
• A double graze expedition in Australia adventure
(YouTube 8min)
• Graze of Aldebaren on waxing crescent moon
with excited commentary (1:07)
• An occultation of the planet Uranus. Note the
slow brightening, since Uranus is 4 arcsec
across, not a point source of light like a star
Why are Grazes Valuable?
• #1. Keeping track of the Earth’s slightly varying rotation
rate. The exact timings of D’s and R’s depends on your
position in space, which depends on how the Earth is
rotating. It changes gradually due to…
• --- tidal friction with the moon,
• --- tidal friction with the sun,
• --- massive Earthquakes can change the rotational
moment of inertia of the Earth and change our rotation
rate in a slight but discontinuous way.
• --- --- Precisely determining the changing relative
coordinate systems fixed to the solar system, the
coordinate system fixed to the stars and galaxies beyond
• --- Using video records, measure the precise
brightnesses of known close double stars, discovery of
new close binary stars
#2: Quantifying subtle
differences between Newtonian
gravity and General Relativity
• Warped space in General Relativity
causes additional precession in the lunar
orbit
• “Frame Dragging” also changes the
moon’s orbit slightly
#3. Mapping the edge regions of
the moon.
• The Watts charts created in the 1940’s
and 1950’s used ground telescopes, but
recently the Kaguya spacecraft (JAXA
Japan) orbited the moon and precisely
photographed and mapped the lunar edge
region.
• Now, timings can be related to the exact
topography of the moon with little
uncertainty.
#4: Precisely Measuring the
Diameter of the Sun
• Timing the appearance/disappearences of
Bailey’s Beads at solar eclipses
• If photometry can be precisely calibrated (too
difficult so far), then we can use the moon as a
fixed yardstick by which to measure the possible
changes in the diameter of the sun by
measuring “Bailey’s Beads” timings during solar
eclipses
• YouTube 6min video of annular Bailey’s Beads
Jan 10, 2010 annular eclipse in India
• YouTube 3 min video of May 10 ’94 Baileys
Beads at annular eclipse
The Problem Here is…
• The photosphere is fuzzy, like the edge of a
cloud. It’s about 350 km in thickness. Since the
moon is 400 times closer, that corresponds to
about 1 km thickness on the moon, and this is
very wide; about the height of typical lunar
mountains.
• It means without highly precise photometry, we
won’t be able to tell the diameter of the sun
using Bailey’s Beads to any better than about
50-100 km at best, and this isn’t very precise. To
this precision, the sun is not changing its size
Bailey’s Beads at 1999 Solar
Eclipse
Bailey’s Beads at Annular Eclipse 2010
An Example: Two Grazes in Carrizo
Plain National Monument in 2014
• The first graze is of 6.0 magnitude ZC 718, a K5
red giant in Gemini, at 8:10pm PDT.
• The path nicely goes across the southern edge
of Soda Lake, and we'll want to set up stations
along the road to the Visitor's Center, with a
particularly favorable station being right at the
gate to Painted Rock (which will be locked) at
the Visitor's Center. The star is red and so will
be brighter than magnitude 6.0 on the video
(which is red-sensitive).
Our Graze Stars, and Nearby Open Star Cluster
The ZC 718 Graze Path, crosses also near Tucson AZ
and Houston, TX where there are active occultation
astronomers who may also get data
Site for ZC 718 Graze Line (blue): Near Goodwin Visitor’s Center
Northern Limit, Dark Side;
Profile for First Graze
2nd Graze only 90 min later, so we’ll have to
hustle a ~10 miles further north to get to it. No
dawdling!
• Pack up your station quickly and follow me to
our second graze site north of Soda Lake.
• We’ll observe it, and only then have a debriefing about what happened for both grazes.
• ZC 726, 10 degrees from northern cusp of the
40% crescent moon, magnitude 7.0 so only
about half as bright as the first graze star.
This northern limit path also crosses very close to
Tucson and may be observed from there.
ZC 726 Graze: On Pole Line Road at 9:35pm
Orange “x” marks primary site along “7 Mile Rd”.
Green is sealevel graze limit. Drive up Soda Lake Rd,
turn right at “7 Mile Rd” and go a ~couple of miles
Profile is very flat; could be
spectacular number of D’s and R’s
How To Record D’s and R’s
• We’ll have ~3 graze stations. Mine, using
video recording. A “student station”, and
then “TA’s Station”, all just a few hundred
yards apart.
• Student station and TA station will use
tape recorders and SW radios and shout
“D!” and “R!” at each event, recording
WWV at 5 Mhz along with the timings.
• Lab Assistant Dave McKulle has a way of
turning the audio recording into a visual
trace so it’ll be much easier to extract
timings for our student graze site, but only
if students chatter is quiet during the graze
period!
• OK – “good luck, Jim!” (from “Mission:
Impossible”