The Bible is very explicit that God created lights to be signs; therefore, the sign of the start of the day should be the presence of light, not the absence of light. Likewise, the sign for the start of the month should also be the presence of light of the moon. Darkness is not a sign that we should follow.

In prior chapters we established that the day begins “early in the morning while it is still dark” and also that the sun rules the day while the moon rules at night with the stars. We can therefore conclude that the day begins when the light of the sun washes out the constellations. This is generally called Nautical Twilight which is when the sun is 12 degrees below the horizon. This is convenient because each day the moon moves 12.2 degrees in the sky so the moon can only appear in Nautical Twilight exactly one morning per month. However, the math indicates that it is possible for the exact moment of the full moon to completely skip Nautical Twilight in edge cases going from 12.1 degrees above horizon one day to 0.1 degrees below the horizon the next.

Since the start of the month must align with the start of a day, we need to be looking for a visible sign at first light that indicates that this first light is also the start of the month. Somewhere on earth the full moon will be exactly 12 degrees above the horizon when the sun is 12 degrees below the horizon. This location becomes the dateline for the month. The full moon should be exactly opposite of the sun; therefore, if the full moon is visible with the stars in the morning less than 12 degrees above the horizon then you know you have started Renewed Moon Day, the first day of the month. This is true because the full moon already happened in an earlier timezone. If it is within the margin of measurement error (+/- 0.5 degrees, 1 moon width) then you are sitting near the new monthly dateline. For 96% of the planet this is unambiguous each month, but for those who are +/- 30 minutes from the dateline it could be difficult to discern which side your location falls on.

While I believe that is the “technical” sign, we have another sign visible at sunset on the 29th day of the month that is a leading indicator. If the sun sets before the moon rises then in 12 hours the moon should be less than 12 degrees above the horizon at sunrise. If the sun and moon are at the horizon at the same time they are almost certainly more than 180 degrees apart because of refraction adds about 1 degree to each and thus at sunrise the moon will be more than 12 degrees above the horizon. Thus if you see sun and moon at the same time at sunset then you will add a 30th day to the month.

This sign is something a 6th grader could follow with a high degree of accuracy. The simwltaious horizon observation is a good rule of thumb that anyone could follow without using any tools, but we also want a 6th grader to be able to observe the true sign in the morning with out any tools.

If you want to observe the sign in the morning, then you need to measure its height above the horizon at nautical twilight. This will give you the most accurate observation of the sign. The moon itself is 0.5 degrees in width in the sky, so the full moon sitting on the top of your fist will place the moon clearly within 10.5 degrees of the horizon. This method will work reliably for ~95% of the months when the moon is clearly less than 12 degrees; however, there will be always be months of ambiguity about once every 2 years when the dateline falls near your location.

You can get more accurate measurements using tools which can be as simple as identifying a fixed spot to observe the moon and marking the expected height on a nearby building. With primitive tools we can can get accuracy of about 0.5 degrees in the sky (one moon width) but there will always be some ambiguity. Furthermore, the moon’s orbit has a slight tilt to it which is why we don’t have solar and lunar eclipses every month, therefore the full moon isn’t always 100% in line with the sun and earth.

With 0.5 degree of measurement ambiguity it means people living on the ground within an area about as wide as a single timezone would have some ambiguity with their direct measurement. However, they still retain the measurement from 29 days earlier which would not be ambiguous at all being approximately 6 degrees above the horizon. One could posit a rule that in case of ambiguity do the opposite of what you did last month. This rule will give you the right answer for ambiguous cases 90% of the time. This in turn means about once every 18 years (known as a Saros cycle) your conclusion will be in error. This means a 99.5% accuracy rate by observation alone.

Every month you get an opportunity to make a new measurement and with these measurements you can calculate the average length of a month and your position in the cycle. Thus with little more math than simple averages over time you can resolve the ambiguous cases accurately 99.9% of the time. Observation is primary and is the ground truth, simple averages of observations is a simple secondary approach.

Unavoidable Ambiguity

In spite of all of these techniques there will always be some degree of ambiguity on the knifes edge of any visible sign. In theory, with enough precision you could calculate the exact moment of the sign such that you get different answers on the two sides of your house. Tools like Stellarium would allow us to do this easily on our phones. At these extreme precisions all calendar systems start to break down and we must rely upon the grace of God if we cross the time-dimension of the sabbath day and profane it in some way.

For starters, the 12 degree twilight line is just a broad convention derived from 1000’s of years of practical application. Nothing in scripture says that 12 degrees is the exact number. The visibility of stars with the moon is subjective, depending upon eyesight and atmospheric conditions thus knowing the exact moment authority changes from the sun to the moon with the stars is beyond the precision we can derive from scripture. Even if we had exact precision from scripture, weather conditions and terrain impact the ability of people to measure and calculate things.

The full moon may be the least ambiguous sign, but it doesn’t overcome the lack of precision in interpretation of scripture. This means that even with our best efforts we may be off on the start of the month by up to 1 day. This in turn means we could be wrong about the objective day of the sabbath by up to 1 day part of the time.

Think about this like a property line around the mountain of God that we are told “do not cross” or the hypothetical question of how thick must your gloves be to safely touch the ark of the covenant? From this perspective it would be wise to treat an ambiguous 30th day as-if it were Renewed Moon day, then if 24 hours go by and the moon is still unambiguously above the horizon the next morning you have a second chance to adjust and keep Renewed Moon day. That said if the moon was within moons width of 12 degrees at the start of the 30th day, then it will be pretty close to the horizon at the start of the next day. Taking measurements near the horizon is more error prone due to terrain and atmospheric refraction and thus your level of ambiguity is unlikely to be reduced.

The issue with getting Renewed Moon day wrong is that you would get the Sabbath Day wrong for the next 4 sabbaths and the only way to stay truly safe is to treat all 4 of these sabbaths as sabbaths. But that doesn’t solve the issue with other feast days like Passover. Hypothetically the ancients wouldn’t have offered two Passover lambs when they were not sure of the day.

In the Millennial Kingdom we will have direct revelation from God about the times and seasons, until then we can only approximate them and we lean on the grace of God and blood of Yeshua to cover our unintentional sins. The key is that the sins must remain unintentional and thus we should still put our best effort forward to discern the times and keep holy space-time holy and only follow consensus of the elders when the start of the month is truly ambiguous. We would error to give the elders authority over the start of most months.