Sky and Telescope - June 2015 - 29

Notional
eclipsing
layer
Sun

ligh

Left: Johann Friedrich Julius
Schmidt (1825-84) directed
Athens Observatory for a quarter century and spent much of
his career drawing, measuring,
and mapping the Moon. He
made 1,400 umbral timings
from 1842 to 1879.
Below: Two great collections
make up the 26,685 eclipse
timings in the new database:
those done by S&T readers, and
a similar number, both modern
and historical, independently
gathered by Byron Soulsby of
Canberra, Australia.
2,500

Timings in database by year
ALL TABLES: S&T: ROGER W. SINNOTT / LEAH TISCIONE

2,000

Number of timings

gleaned more timings from older literature. Especially
important was a long series made by the noted lunar
observer J. F. Julius Schmidt from 1842 to 1879, ﬁ rst in
Germany and later at Athens Observatory in Greece.
When Soulsby died in 2009, David Herald (of the
International Occultation Timing Association) and I
merged and reformatted all these records. This vast
collection serves as a check on just when lunar eclipses
begin and end. It also helps us to assess some quaint
theories about the size and shape of Earth's shadow.
It's well established that Earth's atmosphere makes
the umbra (the shadow's dark central portion) a little
larger than would be expected for an airless Earth. So
lunar eclipses always begin a few minutes early and end
late. This eﬀect was noticed as early as 1687 by French
almanac maker Philippe de la Hire. To allow for it, modern publications such as the U. S. Naval Observatory's
Astronomical Almanac routinely add 2 percent to the
radius of Earth's umbra in their eclipse predictions.
But do careful timings by observers fully support this
procedure? Is 2 percent the right value? No, in fact, as
we'll see later in this article.
There are many further wrinkles. In a 1950 study of
33 lunar eclipses, Czech astronomers Jirí Bouška and
Zdenek Švestka reported that the umbra was somewhat
more swollen if an eclipse occurred within a few days
after a meteor shower, presumably from extra meteoric
dust in the upper atmosphere. Another 57 eclipses, studied in 1954 by František Link and Z. Linkova, appeared
to conﬁrm this ﬁnding. But Herald and I could ﬁnd no
such correlation in our much more extensive data.
We also checked to see if the umbra's size tracks
the 11-year solar cycle in any way. That question suggested itself because, in a 1921 analysis of 150 eclipses by
French astronomer André Danjon, the eclipsed Moon's
brightness and color seemed to be tied to the solar cycle.
In fact, Danjon devised his ﬁve-point L scale for rating
an eclipsed Moon's brightness to aid further study of
this eﬀect. Alas, the database shows no solar-cycle correlation with the size of the umbra at all.

1,500

1,000

500

0
1820

1860

1900

1940

1980

2020

Year

What about major volcanic eruptions? These can loft
many cubic kilometers of dust into the Earth's stratosphere, where it spreads around the globe and persists
for many months. Nine months after a huge eruption
of Mount Agung on Bali, the Moon became as dim as a
star of visual magnitude 4.1 during the legendary total
eclipse of December 30, 1963. But the 615 crater timings
Sky & Telescope received from that event show an umbra
size quite typical of most other eclipses, bright or dark.
Complex eﬀects in our atmosphere inﬂuence the edge of Earth's
shadow. But taken together, their result is simple: the atmosphere acts
like a sharp-edged, opaque shell with a particular depth, the "notional
eclipse-forming layer." It changes depth slightly from one eclipse of
the Moon to another, as measured by careful timings of when craters
(black dot) cross the edge of the shadow's umbra.

Moon
Earth
Umbra
Sunlight

Sk yandTelescope.com June 2015

http://www.SkyandTelescope.com

Sky and Telescope - June 2015

Table of Contents for the Digital Edition of Sky and Telescope - June 2015