The reason for this post is I think I understand it and am seeking reassurance.
I am just starting to study celestial navigation and am trying to get to grips with some of the fundamentals. I need to use the Equation of Time of the Sun to establish Longitude.
Longitude is time and if it were not for the equation of time noon would occur at noon at the Prime Meridian (Greenwich) throughout the year every day. Longitude is established by judging when noon occurs at your location. This is when the Sun reaches its highest point in the sky from your location and is called Local Apparent Noon LAN. When this occurs you consider it is 1200 hrs at your location irrespective of what your watch says. This time (1200) is compared to GMT (UTC) and your longitude can be calculated from the difference, 4 minutes of time for every 1 degree of longitude. Just work out if you are East or West of the Prime Meridian.
Back to the Equation of Time.
When I first read about the Equation of Time I printed a couple of tables off the internet and read how it should be applied. I initially understood the figures given in the tables if stated as + or F (FAST) should be added to UTC. (Today the Equation of Time is + 16 minutes 18 seconds). My new understanding is that the + or F indicates that the Sun is FAST therefore noon (highest point of the sun from your location LAN) will occur before the clock time. This means if you wish to synchronise UTC and Sun time any figures given as + or F should be subtracted from UTC.
So at Greenwich today noon (LAN) will have occured at 16 minutes and 18 seconds to 12.
Conclusion. If Equation of Time is given as + subtract it from UTC. If given as - add it!
Anyone agree/disagree with my conclusion?
Mike.
I suggest you grab a Navigation text, a current one (the EOT changed in the last twenty years). Dutton's if you can find one (it is the one I used as a basic reference for years).
The EOT has to be applied to an observation at Greenwich also. I am uncertain of your meaning on that sentence.
If the Sun is running Fast, LAN then is before local noon.
The use of this is really simple in practice: Grab a text and run through the examples. I could not find any exercises on the internet.
It seems like you are seeking confirmation and clarification on how to use the Equation of Time in celestial navigation. The Equation of Time is a necessary tool in determining local apparent noon (LAN) and subsequently establishing longitude. Let me explain the process and address your conclusion.
To establish longitude using the Equation of Time, you need to observe when the Sun reaches its highest point in the sky from your location, which is known as LAN. Ideally, LAN would occur at noon at the Prime Meridian (Greenwich) throughout the year, but due to the Earth's elliptical orbit and axial tilt, LAN does not always align perfectly with noon UTC (GMT).
Here's the step-by-step process:
1. Determine Local Apparent Noon (LAN): When the Sun reaches its highest point in the sky from your location, consider it 1200 hours at your location, irrespective of what your watch says. This is LAN.
2. Compare LAN to UTC (GMT): Compare the LAN time to GMT (UTC) to calculate the difference.
3. Calculate Longitude: For every 4 minutes of time difference between LAN and UTC, you can calculate 1 degree of longitude. Determine if you are east or west of the Prime Meridian (Greenwich) to correctly apply the difference.
Now, let's address your conclusion about the Equation of Time. The Equation of Time provides corrections for the difference between apparent solar time and mean solar time (represented by UTC). The sign (+ or -) of the Equation of Time indicates whether the Sun is running fast or slow compared to mean solar time.
Based on your understanding, if the Equation of Time is given as + (or F for "Fast"), you initially believed that it should be added to UTC. However, your new understanding is that when the Equation of Time is + (or F), it means the Sun is running fast, and this should be subtracted from UTC to synchronize UTC and Sun time.
Therefore, your conclusion is correct: If the Equation of Time is given as + (or F), it should be subtracted from UTC. If it is given as - (or S for "Slow"), it should be added to UTC.
In celestial navigation, it is helpful to have a reliable navigation textbook, such as Dutton's, to explore examples and exercises that can enhance your understanding and practice. Additionally, note that the Equation of Time has changed over the last twenty years, so it's crucial to refer to an up-to-date resource.
I hope this clarifies the process and addresses your questions. Good luck with your celestial navigation studies!