To do this, we convert the local Standard/Daylight time to Greenwich Mean Time (GMT), also called Universal Time (UT), by adding or subtracting the whole number of hours from our standard meridian to the Prime Meridian several tables in the text (e.g., Tables 14 and 24) show conversion values for various time zones. But with a normal watch we have to calculate the GST.
If we happened to have a watch that keeps GST this would be simple (there used to be ads for such watches in astronomy magazines, as mechanically driven watches can be "regulated" to run fast or slow but "quartz" watches cannot be adjusted in this way). In other words, to find our longitude we need only note the GST at which a star crosses our Meridian, and subtract the star's right ascension from that.
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