Equation of Time.
I have read 2 explanations of the Equation of Time which seem to contradict each other.
Today the Equation of Time is 16 minutes 15 seconds.
I have seen it expressed as 16.15 F. The F means FAST meaning the sun is 16m 15s FAST.
Another explanation advised me to add the 16m 15s to GMT (UTC) before commencing my calculations which incidentally are to establish LONGITUDE using LAN.
Can anyone offer a clearer explanation please.
Mike.
Both are the same. You want for longitude the difference between UT and local time. Local time is determined by a clock, minus the EOT. That is, if the sun is running fast, subtract it from local time. Since you are determing the difference between Local time and UT, adding it to UT is the same.
Longitudetime=Localtime - UT
But if you are observing a sun event, such as meridian, you correct for EOT.
longitudetime= localtime - EOT -UT
or as above
longitudetime= localtime - (UT + EOT)
The Equation of Time (EOT) is a mathematical representation of the discrepancy between mean solar time (the time based on a fictitious average of the Sun's daily motion) and apparent solar time (the time observed by a sundial). The EOT takes into account the elliptical shape of the Earth's orbit around the Sun and the tilt of the Earth's axis.
In your case, the EOT is given as 16 minutes 15 seconds. This means that, at the given time, the Sun is "fast" by 16 minutes and 15 seconds compared to the fictitious average. In other words, if you were to rely solely on a sundial to tell time, the Sun would be ahead of what your clock would indicate.
The expression "16.15 F" means the same thing. The "F" stands for "fast" and indicates that the Sun is ahead of mean solar time by 16 minutes and 15 seconds.
When it comes to calculating longitude using the Local Apparent Noon (LAN) method, it is necessary to consider the EOT. The idea is to determine the difference between the Local Apparent Noon (the time when the Sun crosses the observer's meridian) and Universal Time (UT), which is a standard time scale used worldwide. To account for the EOT, you subtract it from the Local Apparent Noon or add it to UT.
So, to establish longitude using LAN, you have two options:
Option 1: Subtract EOT from Local Apparent Noon (Local Time).
Option 2: Add EOT to Universal Time (UT) before the calculations.
Both options would give you the same result, as they account for the difference between mean solar time and apparent solar time caused by the Equation of Time.
Here's the equation summarizing the two options:
Longitude Time = Local Time - EOT - UT
or
Longitude Time = Local Time - (UT + EOT)
Using either of these equations will help you determine longitude accurately when using the LAN method.