How far can I be from the centerline of the transit path?

Alright!

It was on fire and his glasses were the same
this thing knows if it was tinted
but you know it isn't
to me it is...

Number 9, number 9, number 9, number 9, number 9,
number 9, number 9, number 9, number 9, number 9

- The Beatles
Revolution Number 9

"A number without a specified error is meaningless."

- Dr. Gerry Neugebauer
(Physics 6 +/- 0.3 lab, Suphermore year)

The email alerts have been enhanced by the addition of field "J":

J - how far can I be from the centerline?

For example:

A B----- C----- D-------  E--- F-----  G--  H------ I-------- J---- K L---- M----
m    5.5  8 Jun 22:30:57  48.2 -135.9  320  34.8792  -81.9705   1.9 y -18.8 106.2
m    5.3              57.33                 34.8633  -81.9520
m    6.1  8 Jun 22:30:58  48.2 -135.8  320  34.8315  -81.9150   1.9 y -18.9 106.2

(Unfortunately for you cats who prefer kilometers, I forgot to convert this field from miles- maybe next time.)

The WorldView screen capture below- which I added to using Adobe PhotoElements- shows this predicted pass of the International Space Station (naturally, thunderstorms are also predicted for June 7 - 9):

Field "J" tells me that if I'm 1.9 miles from the center-line, I'll only see the space station graze the moon, rather than pass right through the center. If I'm more than 1.9 miles off the center-line, the ISS will miss the moon completely.

On the center-line, you will see the space station transit the full diameter of the moon; 86.6% of 1.9 miles ( = 1.6 miles) from the center-line, you will see the ISS transit 1/2 of a moon diameter, as depicted in the diagram at the lower left corner, above.

If you're within 50% of the value given in "J," you'll see a good transit- so a useful way to think of field "J" is as the transit path width.

The transit path width depends primarily on the elevation angle of the moon (or sun), but also it depends on the direction to the moon, compared with the direction of the transit path. Unless the transit occurs directly overhead, the area on the earth's surface where the ISS could be seen against the face of the moon will be elliptical- the direction to the moon will determine the orientation of the ellipse to the transit path, and therefore the "width" of the transit path.

For a transit that occurs directly overhead, the space station will be about 240 miles / 385 km overhead, and the transit path width will be only about 2.2 miles / 3.5 km (for a "good" overhead transit, the width should be considered to be 1.1 miles / 1.8 km, corresponding to being 0.6 miles / 0.9 km off the center-line). On the other hand, for a transit that occurs at a low elevation angle (such as a rising or setting moon), the path width may be more than 100 miles!

An overhead transit has the advantage that the distance to the space station is at a minimum, so the space station will be as large as it can be observed, as in Tom Laskowski's superb photo - the disadvantage is that at that distance, it only takes about 0.4 seconds to transit the full diameter of the moon or sun! The greater the distance, the smaller the space station will appear - but you'll be able to blink without missing the transit!

Now, what's this?

A B----- C----- D-------  E--- F-----  G--  H------ I-------- J---- K L---- M----
m    5.5  8 Jun 22:30:57  48.2 -135.9  320  34.8792  -81.9705   1.9 y -18.8 106.2
m    5.3              57.33                 34.8633  -81.9520
m    6.1  8 Jun 22:30:58  48.2 -135.8  320  34.8315  -81.9150   1.9 y -18.9 106.2

Here, I've computed the actual minimum travel distance from my location to the transit path center-line, by interpolating between the adjacent values- this minimum is predicted to occur at 22:30:57.33 Eastern Daylight Time.

Below, I've mapped the transit track using Microsoft Streets & Trips. The blue line marks the center-line, while the parallel red lines lie 3/4 mile to either side - thus, in between the red lines, one could observe a very decent transit... weather permitting.

I entered the predicted postion for 22:30:56 at www.calsky.com, and without much trouble I had it produce the following depiction of what the transit should look like (I added the text, and arrow):

Some people, like Willie Koorts in Capetown, South Africa, have access to a fixed observatory, so they're interested only in transits that can be observed from that fixed location (basically, with an alert radius = 0). By computing the true minimum distance to the transit path center-line, as well as its width, by next week I'll be able to accommodate that scenario.

Wishing you clear skies...