In order to calculate positions of planets and houses, MyAstroChart makes use of Swiss Ephemeris.
Please see the page for the details of the calculation.
Here are some supplemental descriptions.
(1)There are 3 modes of calculation in Swiss Ephemeris. By Swiss Ephemeris, by Moshier and by Full JPL. The calculation in MyAstroChart employs the one by Swiss Ephemeris and provides common astrological position*.
*Apparent body position in geocentric ecliptic polar coordinates relative to the true equinox of the date.
Swiss Ephemeris outputs positions as degree value in floating point decimals. MyAstroChart rounds the second when it displays the position in degree-minute-second format. However, it always cuts down if the round up increases the value of degree. That is, if the value is at 59 minute 59 second and some fraction, the fraction is always cut down for avoiding the increse of degree value.
(3) Retrograde and Stationary
Swiss Ephemeris outputs speed of each planet. MyAstroChart simply denotes R for retrograde if the speed is negative. MyAstroChart does not judge whether the speed means stationary or not, and has no display for the stationary.
(4)Placidus house in high latitude
When Placidus house calculation for high latitude encounters its generic difficulty, Swiss Ephemeris automatically switches to Porphyry house system and MyAstroChart displays the result accordingly.
(5)Drawing Planet symbols
When 2 planets exist in the range of less than 8 degrees, MyAstroChart draws these planetary symbols with different radiuses from the center of the chart in order to avoid overlap of the 2 symbols. There is no meaning in the difference of the radiuses other than that.
(6)Living with the Moon
MyAstroChart calculates each planet position for 198 time points in 33 days for 1 month.
( 198 = 33 days x 6 points/day i.e. every 4 hours )
MyAstroChart derives the time of each event and position of Moon at lunation simply by linear interpolation between
adjacent 2 points obtained above. This method yields enough precision for "Living with the Moon" (less than 1 minute error
to the real time either in time or angle). Then the time is adjusted to the local time specified by Time Zone.
(7)Void of Course Moon
As the Moon goes through each sign, it makes aspects of some sort to other planets. Between its last
major aspect in one sign and its entrance into the next sign, the Moon is said to be void of course.
MyAstroChart calculates the begining time of a void of course based on traditional definition, that is,
as the time when the last major aspect (conjunction, sextile, square, trine and opposition) to one of
the planets (Sun, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto) occurs.
Accordingly, semi-sextile(30 degrees apart) and quincunx(150), and Chiron and true Node are not taken
into account, though the events of their aspects are graphically illustrated as well.
Besides, Semi-square(45) and sesqui-quardrate(135), some astrologers consider they should be included,
are also excluded and not displayed (except for these aspects to the Sun which are just displayed in regard
to the phase of the Moon). There are more other aspects defined in astrology, but
they are not taken at all.
A void of course ends at the time the Moon enters next sign. The Moon goes void of course every couple of
days and the period of a void of course varies from a few seconds to about 2 days long.
Regarding the void of course Moon, some astrologers are indifferent and others pay keen attention.
There are widely varied articles and critical opinions about the significance of and the advice for that period.
Besides, there are some disputes on the definition itself.
MyAstroChart developers have no intention to advocate anything on the void of course.
We display it merely for the reference to get the idea of it.