AAAP Occultation Planner Notes

Here are the notes that explain each of the column headings and the terms used.

Lunar Occultation Calculations

by John Holtz

Full printouts from OCCULT.BAS version 2.3 contain the following information:

Each occultation is represented by one line of data. The columns consist of the following information:

Flamsteed Number: Gives the Flamsteed number of the star if available (blank otherwise).

Bayer Designation: Gives the Bayer designation of the star if available (blank otherwise). The true Bayer designation consists of a letter of the Greek alphabet and optional numerical superscript. Because the text file from which the occultation data is extracted cannot contain the Greek font necessary for the Bayer designation, the following letters will appear. (If you are using a spreadsheet or database to format the calculations, you can "recover" the Greek letter by setting the font of the Bayer Designation column to "Symbol".)

Letter	Greek letter		Letter	Greek Letter
a	α	alpha	n	ν	nu
b	β	beta	x	ξ	xi
g	γ	gamma	o	ο	omicron
d	δ	delta	p	π	pi
e	ε	epsilon	r	ρ	rho
z	ζ	zeta	s	σ	sigma
h	η	eta	t	τ	tau
q	θ	theta	u	υ	upsilon
i	ι	iota	f	φ	phi
k	κ	kappa	c	χ	chi
l	λ	lambda	y	ψ	psi
m	μ	mu	w	ω	omega

Constellation: The official three-letter abbreviation for the constellation if available (blank otherwise). If the occulted object is a planet, satellite, or asteroid, then the name of that object with be in the Constellation column.

Date: The date of the event, in Universal Time (UT, Greenwich England).

Time: The time of the event, in Universal Time (UT, Greenwich England), accurate to a few seconds for Wagman Observatory. Otherwise, watch continuously for a few minutes, depending on the distance from Wagman, or enter your latitude and longitude to adjust to your observing site, or download the CSV format and correct for your location (see A and B description below). Subtract 5 hours from Universal Time (U.T.) to get Eastern Standard Time (E.S.T.); subtract 4 hours to get Eastern Daylight Time (E.D.T.). The event occurs on the previous date if the time is less than 0. The time is expressed in 24-hour military time. Thus, 23:00 is 11 p.m., and so on.

Ph: The phenomenon, either a disappearance (D) or reappearance (R). Disappearances occur on the celestial east (lunar west) side; reappearances occur on the celestial west (lunar east) side. Refer to the CA (cusp angle) to determine if the event is a bright limb or a dark limb event.

SAO or XZ: The Smithsonian Astrophysical Observatory (SAO) number for the star, unless it is preceded by an X in which case it is the U.S. Naval Observatory's XZ number.

mag: The visual magnitude (brightness) of the star. The larger the number, the fainter the star.

Sp: Spectral type, when known. Stars close to the spectrum of the Sun (G2) can be more difficult to see since the color of the light is similar.

Db: If the star is a double star, a one letter code will be given in this column. The codes are as follows:

	A    Listed by Aitken and/or Burnham (ADS, BDS) and thus a visual binary
	B    Close double with third star nearby with separate XZ number
	C    Listed by Innes, Couteau, or other visual observers
	D    Primary of double; secondary has separate XZ entry
	E    Secondary of double; primary has separate XZ entry
	F    Following component
	G    A or C with second star either M, J, U, or V and third star referred to second star
	H    Triple: J, U or V and M
	I    O with secondary either J, U, or V (third star's data referred to secondary)
	J    Single-lined spectroscopic binary (SB1); separation probably <0.01"
	K    U or V, but duplicity doubtful
	L    Triple: J or U, and V; or all V; or all J
	M    Mean position of close pair
	N    North component
	O    Orbit available
	P    Preceding component
	Q    Triple: J or U or V, and O
	R    Triple: O and O
	S    South component
	T    Triple: V and A or C; or all A and/or C
	U    Separation <0.01" (usually SB2)
	V    Separation >0.01", but not visual (occultation, interferometric, or speckle component)
	W    Triple: J or U, and A or C
	X    Probably a close double, not certain
	Y    Triple: K or X, and A or C
	Z    Triple: O and A or C or V or X or L
	$    G except M rather than A or C for 1-2 stars

Pct Ill: Moon's percent sunlit. Positive values (+) are waxing phase, New Moon (0%) to First Quarter phase (50%) to Full (100%). Negative values (-) are waning phase, Full (-100%) to Third Quarter phase (-50%) to New (0%). For events that occur during a lunar eclipse, the percent illuminated is preceded by the letter E, and the value is the percentage of the Moon's diameter not in the umbra. Thus, "E 80" is in the early stages of the partial eclipse (80% of the diameter not immersed in the dark umbra), and "E 0" indicates a total lunar eclipse.

CA: Cusp angle of the event. This is the number of degrees from the northern (N) or southern (S) cusp to the event. During a lunar eclipse, the "cusp" can be in any position, depending on the position of the Moon and the umbra. In these events, the cusp angle is measured from the nearest intersection of the umbra with the Moon's limb. The positive (+) events occur on dark limb (or in the umbra during an eclipse) and are easiest to see; negative (-) events occur on bright limb. See the figures below.

PA: is the position angle measured in degrees. Similar to the position angle for double stars, it is the angle from due north (0°) to the point on the Moon's limb where the Event occurs. 90° is celestial east (to the left), 180° is celestial south, 270° is celestial west (to the right). See left figure below.


Cusp angle during non-eclipse occultation.	Cusp angle during lunar eclipse.

Watt: The Watts angle (in degrees) of the event. This is similar to the position angle (PA), except the Watts angle is measured from the Moon's north pole. The Watts angle can be converted to lunar latitude, and a lunar map can help to identify where the event will occur.

Star Alt: Altitude (in degrees) of the star above the horizon at the time of the event. (Hopefully, the star is above the horizon. Otherwise, you are wasting your time!) 0° indicates the star is on the horizon, 45° is halfway up, and 90° is straight overhead.

Star Az: Azimuth (in degrees) of the star at the time of the event. Azimuths are measured from due north, increasing towards the east (east = 90°, south = 180°, west = 270°).

Sun Alt: Altitude (in degrees) of the sun at the time of the event. Negative values indicate the sun is below the horizon.

A and B: Coefficients that allow the calculated time to be adjusted to a different observing site. To adjust the time to your location follow this procedure: To = Tc + A(lo - lc) + B(fo - fc) where

To = time of event at different observing location
Tc = time of event at calculated location
A and B = coefficients from calculation (minutes of time per degree of longitude or latitude)
lo = longitude at observed location (west longitudes are -)
lc = longitude at calculated location (-79° 48' 45.3" W = -79.8126°)
fo = latitude at observed location
fc = latitude at calculated location (40° 37' 33.6" N = 40.6260°)

These coefficients should provide reasonably accurate adjustments for sites up to 200 miles (about 3° of latitude) from the calculated site. Bare in mind that they are only an approximation. The accuracy decreases as either value becomes larger. When the values are 99.9, this indicates a near grazing occultation condition, and adjustments can not be made.

Scope: Size of the telescope necessary to see the event. This value is based on the program in Sky & Telescope, Jan 1993, pp 89-91. The value is dependent on many factor, some of which are known (magnitude, percent sunlit, cusp angle, etc.) and some unknown (observer's age, steadiness, magnification, experience, etc.). Thus, the scope size is only an approximation.
Just as each observer has his or her own personal equation that indicates their reaction time, each observer will have their own "personal equation" relating to the scope size. You may discover that you can observe occultations predicted to require a 10 inch scope even if you have a 6 inch scope. Thus, you can see more events than predicted. Or, you may discover that you require a larger telescope than predicted to see some events. Once you determine where you fall with respect to the predictions, you can use the scope size to estimate the difficulty of the events.

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