moon phase using JPL Horizons vs HKAS

At the beginning, I think using webgeocalc can determine the moon phase. So I give it try

 angular separation finder:

using moon quarter ( 90 degrees) as example:

2023-08-08 10:29:04.584489 UTC

2023-08-24 09:57:53.976404 UTC

Using webgeocalc can give you only the angular separation, which is different from app geocentric ecliptic longitude difference !!!

Then I try to use JPL Horizons, to find new moon:

in Table Settings, chose Observer Ecliptic Lon, Lat, ie ObsEcLon

then save the results

Do this again with Moon as Target.

Use Excel to subtract ObsEcLon of Earth and Moon.

Found the minimum delta is 09:48

Use illumination Angles, IAU_Moon: >>> The best way

from HKAS 掩星組 post ( timing 時分秒). 

8月8日18時28分25秒（-90°）下弦

8月24日17時57分14秒（+90°）上弦

Other methods:

Pyephem by egoshi

Moontool in visual C (90s)

Tips in stackoverflow

Astrostackexchange. Elong is angle separation , or ecliptic separation?

HKO, NAOJ, IMCCE 天文台等 calculation timing 一般是去到時分.

They are the times when the excess of the Moon's apparent geocentric ecliptic longitude λ_M over the Sun's apparent geocentric ecliptic longitude is 0, 90, 180, or 270 ..., seidelmann 2005, p.478

全年最小(遠地點)新月 是 8 月16日? 可安全觀測( 2 degrees above horizon, too close to Sun)? 不建議!!

USRA

skyandtelescope: 18 Aug

earthsky 18 Aug

inthesky 

Astropixels : phase of moon

perigee and apogee from NAOJ 

Apogee and perigee calculator by csgnetwork, comparable to ELP2000-82 theory

NASA Moon 

Svs nasa dial-a-moon ,  text file for 2023 for every hour

Photography:

Thierry Legault 

S&T crescent moon blog