Delta-T: the plot thickens

Bill J. Gray Apr 9, 2001

Hi folks,

Robert Orso wrote:

"...I don't really like to start a religious discussion here..."

A thought I can heartily second!

The Delta-T story is getting a bit stranger, every day.

In December 1998, I got an e-mail on the subject from Luc Desamore,
a Guide user in Belgium. He pointed out that, when using the ELP-85
lunar theory, an adjustment should be made to Delta-T. He wrote:

"...The correction formula that may be found in the Astronomical Almanac
for 1991 page K8 is:

-0.000091 (n + 26)(year-1955)^2 seconds, n being here -23.8946."

That left me doing a bit of head-scratching. It seemed to me that the
result would be that one expression for Delta-T was used for planetary and
older lunar theories, while a different one should be used for ELP-85. I
did just that, and got results for solar eclipses that match those from
Fred Espenak's "Solar Eclipse Catalog" (which forms the basis of the
"Tables... Solar Eclipse" function in Guide.)

Given some more head-scratching, though, I think Chris Marriott has
it right: it's best to apply a single expression for Delta-T to all
events, one that includes the above correction formula. Doing so does
not affect the actual _occurrence_ of events, but _does_ sometimes
involve their happening at different UT moments. I've made suitable
fixes to Guide, and these will appear the next time I update software
on the Web site.

However, this is not the end of the story. As I speculated in my
previous post, more research has resulted in yet further changes in
estimates for Delta-T. See

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/deltaT2.html

"...More recently, Stephenson presents a new analysis of most if not
all known solar and lunar eclipses that occurred during the period -700
to +1600 (Historical Eclipses and Earth's Rotation, 1997)..."

Using the values shown there, Delta-T for the "Christmas Star" event
we've discussed is about +10600 seconds, about 800 seconds greater than
the value I previously used. I assume this means the secular acceleration
of the moon is bounced back up, but I need to check on this.

-- Bill