# How to convert sidereal time to local time?

I came across something that won't let me go, but no matter how much I calculate (which I do not like), it doesn't seem to work. Also: I've used so much apps and links already to convert, but it doesn't work in that direction. You see, local time converting to sidereal time = not that hard.

I need to know the local 'normal' time of my local sidereal time. The sidereal time is exactly 13.30h, from that I would like to know the normal local time.

• The time zone = Brussels (CET)
• Olson time zone Europe/Brussels
• Time difference: same as Amsterdam and Europe UTC+1
• no summer time at the moment (and what if that was the case, what would the local time be then?)
• Latitude: 50,929315°
• Longitude: 5,337367°
• Address: Grote Markt 18, 3500 Hasselt Belgium.

The reason I need to know this has to do with a sort of awareness I found in ancient writings and books. It has extra power than to meditate, and I would like very much to try this out.

But of course, I need to know the local time then.

• How far back is "ancient" and how accurate do you need the conversion to be ? The Earth's rotation with respect to the stars (sidereal time in essence) gets increasingly uncertain as you go beyond approx. 750 BCE (see errorbars on the left side of this plot showing the difference between time given by the Sun/Earth rotation and atomic time/UTC extended back in time) – astrosnapper Feb 5 '19 at 21:42
• How does the ancient text reference the time? Is it by (a) the sidereal time, (b) the constellation on the meridian and you converted that to 13.3 hour, or (c) by a specific date and time and you converted it to 13.3 hour? If (a), then precession may be a problem. If (b), then you should be okay. If (c), then astrosnapper's comment is correct and you will not be able to convert a date/time that was long ago into an accurate sidereal time. – JohnHoltz Feb 6 '19 at 4:17

$$local\;time\;for\;13.30\;LST = t+\frac {13.30-LST\;at\;current\;date\;and\;time\;t}{1.0027379}$$
For example, at 3:00 local time (2:00 UTC) on Feb 6, 2019, the LST=11:25:02 (11.4172 hours) at 5 deg 20' 15" east longitude (5.3375 deg). The above formula gives the time for 13.30 hours LST as $$=3:00+\frac{13.30-11.4172}{1.0027379}$$ which equals 4.8777 hours local time (4:52:40). Plugging that time (converted to UTC if necessary) back into an app confirms the LST is 13:18:00 (13.30 hours).