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This page was last updated on 10 December, 2015.
Calculate conjunction

The italic part is just background info you can skip if you like.

- Always measure from the center of the sun and the center of the moon.
- If the moon isn’t full guess it’s center as if it was full.
- For best accuracy average a few measurements.
- Always measure after full moon. Fourth week is most practical.

- 29.530588853 days in a synodic month. Click
- Times 24 is 708.734132472 hours in a month.
- Divided by 360 is 1.968705923533333 hours per degree.
- Below I'll call that number HourAngle (HA).
- A degree is roughly a centimeter.

- 27.321661547 days in a sidereal month. Click
- Times 24 is 655.719877128 hours in a month.
- Divided by 360 is 1.821444103133333 hours per degree.
- Below I'll call that number HourAngle (HA).
- A degree is roughly a centimeter.

The difference between synodic an sidereal. Click

I’ll use the sidereal month.

A lot of digits but don’t be fooled into thinking they will come even close to NASA accuracy because the measurement methods shown below are very inaccurate. Use as many digits as you like but 2 digits (HA=1.97) is enough for most purposes. If you don’t have a calculator and/or aren’t good at math using HA=2 will do the job.

All below measurements are between the estimated centers of sun and moon.
- When the sun is only partly visible estimate the center of the sun.
- Estimate the center of the moon as if the crescent is round full moon.

Method A - Protractor
- Go outside just before sunrise with a protractor.
- Measure the angle between the horizon and center of the moon.
- Multiply result by HA  for the hours until the next conjunction.

Method B - Measuring tape
- Go outside just before sunrise with a centimeter grade measuring tape.
- Measure the distance between moon and sun with your arms stretched.
- Multiply result by HA  for the hours until the next conjunction.
- If using inches multiply by 5
- You now have the hours until the next conjunction.

Method C - With bare hands
For this method you only need your hands. Because of the body proportions it doesn’t matter how big or small you are because small hands are attached to shorter arms. Because the measurements are done with stretched arms the fingers look quite the same size for everyone*.
Go outside just before sunrise and don’t forget your hands :-)
The measurements of your hands:
- Pinky finger 1 degree.
- Index, middle and ring fingers together are 5 degrees. (About 1.7 each)
- Clenched fist without thumb is 10 degrees.
- All 4 fingers spread as wide as possible are 15 degrees.
- Whole hand spread as wide as possible are 25 degrees.
Multiply by HA for the the hours remaining until conjunction.

*=All numbers assume a certain average size of hands and fingers. You can improve accuracy by measuring your hand and fingers. 1cmxHA=hours.

Method D - No calculations, no tools
- 29 day month if the moon that rises around sunset of the 14th is full.
- 30 day month if the moon that rises around sunset of the 14th is not full.

Method E - No calculations, no tools
This is great method to verify/combine with method D

End of the old month
- Check the sky shortly after sunrise on the 29th.
- No visible crescent? Next day is NMD. If visible wait another day.
- Celebrate NMD.
- Crescents gives an indication if the month will have 29 or 30 days.
- The smaller the crescent the shorter the time it's visible in the sky.

Method F - First crescent
If the first crescent in the west is seen for more than an hour it will be a 30 day month.

Method G - First quarter
When the first quarter, which is right above our head the night before Sabbath, is a little past half it will be a 29 day month. If it doesn't reach half during the night it likely will be a 30 day month.

Method H - Last crescent
If the last crescent is seen around sunrise of the 28th, it will be a 29 day month. If it's seen in the morning of the 29th it will be a 30 day month.

Start of new month
- Check the sky about 30 mins after sunset.
- If a crescent is visible the next day will be a working day. The current day is a NMD.
- The smaller the crescent the shorter the time it's visible in the sky.

Bad weather conditions
If the crescent is in the sky but not visible due weather conditions keep an extra NMD. If later the weather gets better just adjust the days if needed. Meaning your workweek may be  a day shorter.
There are never more than 2 New moon days in a row.

Use which method?
The computer method is the most accurate and can be a solution for periods of bad weather or for those who live in a area with a blocked horizon (buildings, trees, mountains) You can also lookup conjunctions for any date which could be handy for planning your work schedule. Some will reject this method because they feel God commanded to watch the sky.

Sighting has the advantage you don't need tools and calculations of any sort.

The measurement methods have the advantage over the sighting method that you can predict a few days into the future. That could be handy to avoid problems if the sky gets clouded on the day you most need a clear sky. That said an experienced sighter can also predict by the size of the crescent.

Or a combination. Of course it's also a matter of preference. It could be good to sight the moon but use software to verify your findings until you feel experienced enough to rely on your own skills.

Full moon - verification only

When you watch the sky regularly you will notice there are three days with a full moon.

The time you watch the sky is of importance too.

14:40, 28 June 2018:  99.81% full moon.

22:40, 28 June 2018:  99.48% full moon.

Keep in mind that the ancients only had their naked eye to spot the moon. While the two examples above without a doubt are not full moons because they are not at exactly 100%, the eye isn’t able to see.

Even if we could see the difference between 100% and 99.5% there needs to be an agreement at what time the spotting should be done. If everybody spots at a different time on the same day, everybody would report a different fullness of the moon.

So there needs to be a rule…

The time of determining the full moon is during the full period of evening twilight. That’s the period of time from the sun starts setting until the sun is complexly gone.

That’s the Biblical full moon. Other moons fail at least on at least one of the three criteria. Just in case you didn’t spot it, the third one is that the moon looks full/round.

I must add watching the full moon is only a verification. It never changes the day of the month. The month starts at the new moon and that’s it. If a mistake was made declaring the start of the month and that mistake is exposed by the full moon, then no day(s) are added/removed to compensate for that error. But it can be a good learning tool so that spotting the new moon becomes more accurate every month.

Yeah, I know there will be always a gray area because we simply can’t differentiate between 100% and 99.87% full. So there are times mistakes are made. Or due to weather conditions sighting was very troublesome at best

Personally I don’t think such a honest mistake will anger theFather. He’s  very well aware of the limitations of our eyes and brain. There is a huge difference between accidentally making a mistake and totally rejecting the moon for all calendar related things.