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terekgabor
Senior Member

Joined: 02/01/2026
Location: Hungary
Posts: 161

Posted: 04:43pm 01 Nov 2026

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Hello Everybody,

I am using ASTRO command to get moonrise and moonset moments in a program.
I found when I set the correct time and date (for example 01-11-2026 17:00:00) I get the moonset and moonrise value 1 hour less then the real.
Now in Hungary UTC+1. Does it mean ASTRO is calculating in UTC? But anyway I set the right GPS coordinates and local time in LOCATION command.
Same for sunset and sunrise. May I miss something?

G@bor

matherp
Guru

Joined: 11/12/2012
Location: United Kingdom
Posts: 12101

Posted: 07:32pm 01 Nov 2026

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ASTRO works in UTC. The time you give LOCATION is taken as UTC (Universal Time), not local time, because the sidereal time the calculation needs is defined from UTC. In Hungary (UTC+1 now, UTC+2 in summer), 17:00 local is 16:00 UTC. If you give LOCATION 17:00, ASTRO calculates the sky for 18:00 your time, so every rise or set time you find comes out one hour early.

Convert to UTC before LOCATION, and back again for the results:

tz = 1 ' hours ahead of UTC: 1 in winter, 2 in summer
t = Epoch("01-11-2026 17:00:00") - tz * 3600
Location DateTime$(t), 47.50, 19.04 ' Budapest: latitude, longitude (East positive)
Astro Moon alt, az
When your search finds the moment the altitude crosses the horizon, add tz * 3600 to that epoch time and pass it to DateTime$ to get the local time.

Make sure the longitude is positive for East. The documentation didn't say the time must be UTC; it will in the next release.

terekgabor
Senior Member

Joined: 02/01/2026
Location: Hungary
Posts: 161

Posted: 07:54pm 01 Nov 2026

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Thank you Peter! Now it’s clear.

G@bor

terekgabor
Senior Member

Joined: 02/01/2026
Location: Hungary
Posts: 161

Posted: 08:00pm 01 Nov 2026

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Is it the same with STAR? (If I have GPS signal live.)

G@bor

matherp
Guru

Joined: 11/12/2012
Location: United Kingdom
Posts: 12101

Posted: 10:39pm 01 Nov 2026

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STAR takes no time argument. It always uses the GPS's own UTC time, the copy saved before the OPEN … AS GPS offset is applied

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