Caption: Heliacal risings explicated and illustrated.

Features:

1. Probably precisely because the horizon provided a measurement tool, horizon phenomena (e.g., the risings and settings of the Sun, the Moon, and bright stars were often of key interest to prehistoric peoples, ancient historistoric peoples, and even later peoples who frequently got up at about sunrise because there was light---or to milk the cows---they were tied by lifestyle to daylight---this is NOT us.

2. Among many other things, heliacal risings of bright star were often noted since they marked times of the solar year = 365.2421897 days (J2000).

3. Recall, the Sun moves eastward continually relative to the stars by ∼ 1 degree per day along the ecliptic---and therefore the stars move westward relative to the Sun by ∼ 1 degree per day.

   So if a star near the ecliptic is about 1 degree east of the Sun today, it rises just after the Sun and will usually be invisible to the naked eye lost in the sky brightness due to atmosphere-scattered sunlight: i.e., daylight.

   Tomorrow, the star will rise approximately with the Sun---this is its nearly exact heliacal rising.

   Heliacal rising just means rising with the Sun

   The day after the exact heliacal rising, the star will rise before the Sun and if sufficiently bright can be observed: this is the observable heliacal rising or just the heliacal rising in common parlance.

4. Actually, any rising of an astronomical object (e.g., a star or constellation) near sunrise especially if it is visible to the naked eye is usually considered a heliacal rising.

5. After its heliacal rising, an astronomical object fixed on the celestial sphere will rise earlier and earlier in the night until it rises at sunset---and then it will rise earlier and earlier in the daytime (setting closer and closer to sunset) until it rises with sunrise (and sets near sunset), and so is back to its heliacal rising phase.

   How long is a star NOT visible in most of the night sky because it is up mostly in the daytime? This depends on many factors including how exacting the observer is. Yours truly would guess of order 20 days for a pretty, but NOT extremely, exacting observer.

6. Yours truly mnemonicks process of the Sun moving continually eastward relative to the fixed stars by reciting mnemonic "the stars rise earlier and earlier every day".

   Mistake alerts for image:

   1. I should have written "stars just west of the Sun" above the horizon line and "stars just east of the Sun" below it.
   2. It's NOT the Sun's glare in most cases that makes the stars invisible after sunrise, it is the sky brightness of daylight.

   Credit/Permission: © David Jeffery, 2003 / Own work.
   Image link: Itself.
   Local file: local link: heliacal_rising.html.
   File: Archaeoastronomy file: heliacal_rising.html.
   

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