Planetary phases

[Roberto]

I think this is a question for Ed. How PW calculate the planetary phases for Mercury?
In Roberto Benigni chart (Roberto Remigio Benigni born on   27 October 1952 at 13:00 (= 1:00 PM ) Place   Castiglion Fiorentino, Italy, 43n20, 11e55) PD gibe Mercury visible, Phasis a classical astrology program invisible. from Wikipedia "Mercury and Venus are visible only in twilight hours because their orbits are interior to that of Earth. Venus is the third-brightest object in the sky and the most prominent planet. Mercury is more difficult to see due to its proximity to the Sun. Lengthy twilight and an extremely low angle at maximum elongations make optical filters necessary to see Mercury from extreme polar locations" ( https://en.wikipedia.org/wiki/Classical_planet#:~:text=Mercury%20and%20Venus%20are%20visible,its%20proximity%20to%20the%20Sun. ). about Twilight we have different definition https://en.wikipedia.org/wiki/Twilight

[Roberto]

Checked also with another prg, the first day that Mercury was visible was 07/11

[ABer]

Hi Roberto - this is from the help file.

Visibility

V = Visible
I = Invisible

Planets close to the Sun (within fifteen degrees) are identified as invisible. If the planet is beyond the Sun's longitude
(+/- fifteen degrees) it is considered to be visible. This is a rough guide, and different planets have different
thresholds for visibility.

Of course, in this chart, Mercury is not actually visible  because the Sun has not set. However, it is beyond the threshold of 15 degrees so distant from the Sun and symbolically visible (not under the beams). To calculate whether Mercury is actually visible (can be seen with the naked eye) before dawn or after sunset is an extremely complex task because you have to not only considered distance from the Sun, but the planet's latitude (north or south of the ecliptic) and the latitude of place, and the Sun's relationship to the horizon.

I've just checked some traditional sources (e.g. Al Biruni) and interestingly he gives varying amounts of longitude for being under the beams (Mercury and Venus - 12 degrees, Jupiter and Saturn - 15 degrees, Mars - 18 degrees). I might change this script to reflect this. However, I don't propose to do the code for actual visibility as it would be far too complicated.

I guess you could argue I'm confusing being free of the Sun's beams with being visible, but I have to draw a line somewhere.

Thanks for the posting. Ed

[ABer]

Hi Roberto - there might be a workaround for this that's much easier: calculate the altitude of the Sun and Mercury/Venus. If the altitude of the Sun < -17 degrees and the altitude of the Mercury/Venus > 0 degrees then we could assume that the two planets are technically visible.

It might be worth a try.

Ed

[Roberto]

Hi Ed
thanks for the explanation. I hope that we can come, in the future, with an easy solution close to the visibility with naked eyes. Classical users are very picky about this argument. For now, thanks again for your work on PD, for me is one of the best software around.

[AB]

Hi there. I'm an amateur 'astronomer' and there is a very easy rule that can be employed with both planets and the sun. And the same goes for all the others. You hinted at it in your last comment. And it all relates to the Apparent Magnitude of a planet when close to the sun, and their height above or below the horizon in terms of altitude.
I'll just give a list and then you will decide if it works for you. This is based on my observations and the literature I reviewed on the subject when I educated myself about observing the night sky.
The dawn and dusk has three phases, and they all depend on the negative altitude of the Sun,
The indigo hours, where the Sun is -18 degrees below the horizon (they begin then), the blue hours when he is -12 degrees below, and the golden hors when he is -6 degrees below the horizon. For all planets to be observable without interference from trees and such it is desirable for them to be at at least +2 degrees altitude. Now depending on their apparent magnitude we can construct a list that will hold generally true (with some variation) across the year and longer planetary cycles.
And this is the list:
Moon: she above the horizon +2 degrees the sun below the horizon -4 degrees.
Venus: +2, the sun -4
Jupiter: +2, sun -5
Mercury: +2, the sun -7.5 (in the case of Mercury it actually ranges between -6 and -9 depending on the solar phase).
Saturn: +2, sun -8
Mars: +2, sun -9
Uranos: +10, sun -12
Neptune: +12, sun -18 (theoretical, based on the Bortle scale which allows for true keen eyes and exceptional sky viewing locations to observe Neptune with the naked eye, with limits pushed to apparent magnitude +8.5 or so).

As you can see this list allows for the brightest objects, the crescent moon, and Venus to be observed helically rising or setting in the golden hours, along with Jupiter, especially when he is in his perigee. The next triad, Mercury and Saturn and Mars to be observed in the blue hours, and in the case of Uranos the limit is the indigo hours and for Neptune the limit is the indigo hour itself.

If you use the altitude of the sun and the object in question as a basis for being under the rays and the helical rising and setting, it's much more realistic than the simple ecliptic longitude rule which can be cheated drastically by the inclination of the ecliptic and the ecliptic latitude of the object in question.

[ABer]

Thanks - that's really helpful. I was working towards something like that but I got side-tracked onto other projects. I will try and implement this over the weekend. Most of the code's in place. I just need to finish it off. Ed

[Jean]

Thanks for contributing!

Hi there. I'm an amateur 'astronomer' and there is a very easy rule that can be employed with both planets and the sun.

[AB]

My pleasure! I'm glad if I could help.
There are more specific rules but those would be probably difficult to implement in terms of coding.
I can give you guys a few examples just as a curiosity.
Venus is much brighter when she helically rises as a morning start and when she sets as an evening star compared to when she sets as a morning start and when she rises as an evening star. That is, Venus is much brighter at inferior conjunction than at superior conjunction, as such her heliacal rise and setting around this time happens earlier and later than when she is close to her superior conjunction (in terms of altitude difference between her and the sun).
Mercury is much much much brighter at superior conjunction than at the inferior one, which naturally reduces the difference between him and the sun in terms of altitude needed for him to be visible at superior conjunction.
The Moon is larger and brighter at perigee.
Mars is rather dim and small in terms of apparent diameter (as small as Uranos and Mercury) as he gets closer to the sun with a difference of a 60 degrees in longitude or so, so that's why he needs that rather large difference in altitude between him and the sun to be barely visible. Except once ever 15 or 17 years when he is at his perigee (or more precisely perihelion) and then he is slightly easier to observe as he rises in the morning or just before he sets in the evening.
Saturn's apparent diameter and apparent magnitude varies based on two factors, his closeness to the sun and earth and his rings. He will rise earlier from the sunlight and set later when he is around Gemini, that's when he is at his brightens. He will rise later and set earlier when he is around Virgo and across from her in Pisces. When he is around the end of Sagittarius he is between these two.
Uranos and Neptune's magnitude and apparent size varies less, but both are naturally easier to observe at their perihelion.

[Jean]

Fascinating. When i was growing up i loved reading about the planets in encyclopedia and other books, though i never got to that technical astronomy level.

[Roberto]

Thanks AB very appreciated your contribution

[ABer]

Hi there - I am releasing an update to this module today:

v. 4.05. - 21 May 2021: improved visibility definitions and USB definitions, added altitude listing, improved layout

From the help:

Sun's Beams

USB = Under the Sun's Beams

Two definitions of being under the Sun's beams are offered.

One is a fixed definition where any planet between 8 and 15 degrees of the Sun, measured in longitude, is considered to be 'under the Sun's beams'. A planet beyond 15 degrees of longitude beyond the Sun is considered to be 'free of beams'.

A planet less than 8 degrees from the Sun, but beyond about 16 minutes from the Sun, is considered to be combust. In general this is taken as a malefic state for the planet.

A planet within +/- 16 minutes of the Sun is called 'cazimi', a benefic state where the planet is strongly dignified by its very close connection with the Sun.

A second option is included, where the various distances from the Sun vary as follows:

Free of Beams - Moon (12 degrees), Mercury (12 degrees), Venus (12 degrees), Mars (18 degrees), Jupiter (15 degrees), Saturn (15 degrees).
Under the Beams - Moon (7 degrees), all other planets (6 degrees), except Mercury and Venus when oriental and direct (7 degrees).

The definition of cazimi does not vary.

This option is taken from Al Biruni (The Book of Instructions in the Elements of the Art of Astrology), 481-484.

Visibility

V = Visible
I = Invisible

Various definitions of visibility are used in this module. The first is simply a planet being 'free of the Beams' whether it can be seen visually or not. The symbolic implication of being free of the beams is that the planet has the potential to be seen, not being overwhelmed by the power of the light of the Sun.

Planets close to the Sun (within fifteen degrees) are identified as invisible. If the planet is beyond the Sun's longitude (+/- fifteen degrees) it is considered to be visible. This is a rough guide, and different planets have different thresholds for visibility because of their varying magnitudes and relationship to their synodic cycle with the Sun.

A second definition of visibility may be chosen. This considers whether a planet can actually be seen in the night sky. This is calculated by checking if the Sun is below the horizon, and whether any of the seven traditional planets are at least 2 degrees of altitude above the horizon. Due to the varying magnitudes of the planets, the negative altitude of the Sun is varied in the calculation as follows:

Moon: the sun below the horizon -4 degrees
Venus: the sun -4
Jupiter: sun -5
Mercury: the sun -8 (in the case of Mercury it actually ranges between -6 and -9 depending on the solar phase).
Saturn: sun -8
Mars: sun -9

Essentially if a planet is dim in the night sky, the more difficult it is to see the planet, and the darker the night sky needs to be before the visibility threshold is reached.

The application of 2 degrees of altitude allows for the obscuration of the light of the planet due to atmospheric extinction. The closer a planet lies to the horizon, the more its light is dimmed by its passage through the earth's atmosphere.

For further information see: https://jcremers.com/forum/index.php/topic,147.0.html (Thanks to forum member AB for the information.)

A listing of planetary altitude for the visible planets has been included.

Thanks. Ed

[Roberto]

Thanks very much, Ed very appreciated the improvement of the function.

Cheers

Roberto

[ABer]

You're welcome Roberto. By the way I have worked out how to calculate the date of the appearance of visible planets from out of the Sun's rays. I'll build this into the module. Ed

[Roberto]

Well done Ed, I knew that you will find a solution.  All the best.

Cheers

Roberto