Caption: Heliacal risings explicated and illustrated.
Features:
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.
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.
Credit/Permission: ©
David Jeffery,
2003 / Own work.
Image link: Itself.
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