Moon Phases and Eclipses

by Phill Edwards

Moon Phases

To calculate the Moon phase, first calculate the longitude of the Earth λE and the longitude of the Moon λM for the date. Then calculate the lunar solar elongation D = λE - λM. The Moon phase can then be determined from the phase angle as shown in the table.

Phase Angle Phase
D = 0 New Moon
0 < D < 90 Waxing Crescent
D = 90 First Quarter
90 < D < 180 Waxing Gibbous
D = 180 Full Moon
180 < D < 270 Waning Gibbous
D = 270 Last Quarter
00 < D < 180 Waning Crescent



For a Solar eclipse to occur it needs to be close to a New Moon. New and Full Moons where the Sun, Earth and Moon are aligned are called syzygy. There is not an eclipse at every New Moon because the Moon's orbit is inclined at about 5 to the ecliptic. So, for a Solar eclipse to occur the lunar latitude at the syzygy βsyz needs to be within a certain range.
The angular radius of the Sun ρS = RS/rE
The angular radius of the Moon ρM = RS/rM
The maximum parallax of the Moon δM = RE/rM
An eclipse, either total, annular or partial, occurs if |βsyz| < δM + ρM + ρS
A total eclipse occurs if ρM > ρS and |βsyz| < δM + ρM - ρS
An annular eclipse occurs if ρS > ρM and |βsyz| < δM + ρS - ρM

Lunar Parallax

The Moon is sufficiently large that the Moon as seen from a hypothetical viewer at the centre of the Earth (geocentric) will appear in a different postion to an observer on the Earth's surface (topocentric). In fact observers at different latitudes on the Earth's surface will see the Moon at a different position. This effect is due to parallax. There is also a similar, but much smaller parallax offset for the Sun. The Solar parallax is considered a derived astronomical constant with a value of 8".794. Only the sine of this angle is required for calculations.

Given the following parameters:

We need to calculate the topocentric values λ', β' and s'.

Then: S = ρ sin ψ C = ρ cos ψ N = cos λ cos β C sin π sin θ

tan λ' = sin λ cos β sin π ( S sin ε + C cos ε sin θ ) N

tan β' = cos λ' ( sin β sin π ( S cos ε C sin ε sin θ )) N

sin s' = cos λ' cos β' sin s N

Eclipses Revisited


At the new moon syzygy D = 0
By the cosine rule rMS2 = rS2 + rM2 - 2rSrMcosβ
r UM = R M r MS R S R M

Besselian Elements

Besselian Elements were introduced in 1824 by the Prussian astronomer and meathematician Frederick Bessel. They are a good method of calculating the characteristics and locations of an eclipse which are still valid. The principle is to define a fundamental plane, which is a plane which passes through the Earth's centre and is normal to the Sun-Moon centre line. The x axis point East, the y axis point North and the z axis point towards the Sun. There are six Besselian Elements which are:

The Besselian Elements are normally calculated accurately at five hourly intervals and then a least squares interpolation is performed. This may not be necessary using modern computers. The elements are calculated for the nearest hour to the maximum eclipse and for the two hours before and after the greatest eclipse.

Let r be the Earth-Moon vector and R be the Sun-Earth vector and k = (0, 0, 1) is the unit vector pointing North.
The unit vector K which is normal to the fundamental plane, pointing in the direction of right ascension a and declination b is:

K = R - r | R - r | = ( cos d cos a , cos d sin a , sin d )

The other two unit vectors in the fundamental plane are the unit vectors created from the cross products:

I = K x k | K x k | = ( - sin a , cos a , 0 ) J = I x K | I x K | = ( - cos a sin d , - sin a sin d , cos d )

The Besselian Elements x, y and d plus the z coordinate are:

x = r . I y = r . J z = r . K sin d = K . k

The hour of the shadow axis μ in terms of the right ascension a and Grenwich Sidereal Time Ts is:

μ = Ts - a

The penumbral and umbral cone angles f1 and f2 are:

sin f1 = R S + R M | R - r | sin f2 = R S - R M | R - r |

The penumbral and umbral cone vertices c1 and c2 are:

c1 = z + R M R E cosec f1 c2 = z - R M R E cosec f2

The penumbral and umbral cone radii in the fundamental plane L1 and L2 are:

L1 = c1 tan f1
L2 = c2 tan f2

Besselian to Geocentric Coordinates

Given the Earth's equatorial radius a = 6378.1km and the Earth's polar radius b = 6356.8. Then the flattening f and the ellipticity e are defined as:

f=a-bb e=1- b2a2

Given an observer's longitude λ, latitude φ and distance from the centre of the Earth ρ then the observer's Geocentric position ρG is:

ρG=ρ (cosφcosλ ,cosφsinλ ,sinφ)

The observer's location in the fundamental plane is:

ρF= (ξ,η,ζ)

The x, y and z axis rotation matrices are R1, R2 and R3.

Then the relationship between observer coordinates is:

ρF= R1 (π2-d) R3 (π2-μ) ρG