Behind The Scenes: Reinventing The Wheel

Blog post from Museum Educator Alex Malischostak. To see more posts from Alex, click here.

One of the exciting interactives in the new Jews in Space: Members of the Tribe in Orbit exhibit is the calendar wheel. This wheel allows visitors to look at dates from the Jewish calendar year of 5781 and see what the equivalent dates would be in the standard (Gregorian) calendar and what the phase of the moon would be for those dates. Though this wheel is a simple enough interactive to use, the process to create it was far from it. In this post, I am going to share the methodology I used to align the calendars and some of the challenges that came from trying to align two different calendar systems with the lunar cycle. In the next post, my colleague Marisa will explain the process of designing the wheel itself and some of the creative and functional choices that were made.

When I approached the task of lining up the calendars, I first looked at a template from NASA that showed how the lunar phases corresponded with the 2020 calendar year. I figured I only needed to pop in the Hebrew dates next to the 2020 dates and then follow the pattern for the 2021 dates but I soon encountered a few complications from the different calendar systems.

First problem: there are significantly less dates in a lunar cycle (about 354 days) than a solar cycle (about 364 ¼ days). There are simply less days in 5781 than in a standard calendar year so there would be a chunk of days from either 2020 or 2021 in September that would not be included. For example, if someone wanted to see what the Hebrew date would be on their birthday on September 13, they would not find it on the wheel because the year 5781 goes from September 18, 2020 until September 6, 2021. So what to do about those pesky 12 days? We would have to include either extra days from the end of 5780 or the start of 5782. We decided to add the days at the end of 5780 so the wheel technically goes from the 14th of Elul (the 12th Hebrew month) 5780 to the end of Elul 5781. The tricky part of this is when you look at the wheel window; you will see the dates go from the end of Elul (5781) to the middle of Elul (5780) as you turn the wheel from the end of the cycle back to the beginning.

A more complicated problem is breaking down the days of each calendar to the corresponding lunar cycle. This problem rises from the definition of what constitutes a new day. Should be simple, right? Well, according to Jewish law, a new day begins at sundown so a Hebrew calendar date actually spans across two Gregorian dates. For example, the 1st day of Tishrei, 5781 began on the evening of Septmeber 18 and ended on the evening of September 19. Even more complicated, the calendars are aligned based on the eight lunar events- new moon, waxing crescent, 1st quarter, waxing gibbous, full moon, waning gibbous, 3rd quarter, and waning crescent. However, sometimes an event occurs locally at night and other times during the day. So, which Hebrew date do I give for these events so the calendar can progress consistently? For example, when the new moon occurs at 5:07 am on November 15, 2020 do I say that the Hebrew date is 28th of Cheshvan (evening of November 14-evening of November 15) or the 29th of Cheshvan (beginning in the evening of November 15th)?

In order to keep the dates consistently aligned throughout the year, I decided to go by the Gregorian and Hebrew dates for the evening of the lunar event. So for example, even though the “official” new moon in the previous example will be November 15, I put the date as November 14 and Cheshvan 28 with the waxing crescent cycle starting on November 15/Cheshvan 29. To further keep everything consistent, I decided that the cutoff for the dates would be 6:00 am. Any event that takes place between 12:00-5:59 am is listed as the previous date and every event 6:00am –11:59pm is listed for that day with the Hebrew date that starts that evening.

One more point of interest that I want to highlight is how the Hebrew calendar months line up with each lunar cycle. Each Hebrew month is tied to the phases of the moon. The start of a new month in Hebrew is called Rosh Chodesh (literally, “head of the month”) and corresponds to the new moon with the middle of the month roughly aligning with the full moon. However, as you look through the calendar wheel, you will notice that sometimes Rosh Chodesh does not start until the waxing crescent phase with the new moon occurring in the last few days of the previous month. In my research, I discovered two reasons for why Rosh Chodesh does not always fall on the official new moon. The first reason is that Jewish law and NASA have different definitions of what makes a new moon. Scientifically, the new moon is exactly when the moon is invisible to the Earth when the sun and moon are aligned. However, during the 2nd Temple period of Jewish history (approximately 400 BCE-70 CE), a new moon needed to be observed by 2 witnesses in order to trigger the start of a new month. But wait, how can you observe a phenomenon that is invisible!? According to tradition, the new moon begins when the moon is observed at the smallest point of a new cycle, or what would technically be the first evening of the waxing crescent. The second reason for the variation is that in approximately 400 CE, the Jewish calendar was standardized into a 19-year cycle independent of astronomical observations. As part of the standardization process, certain calendar months were given a flex option of either 29 days or 30 days in order for certain holidays like Passover and Yom Kippur to only fall on certain days of the week. As a result, on certain months, Rosh Chodesh will not fall on the day of the actual new moon but a little earlier or a little later.

Compiling the spreadsheet with Hebrew dates, Gregorian dates, and their corresponding lunar phases proved a trickier assignment than I originally thought with many gray areas but it was also fun to explore the weird quirks of our dating systems. This process made me appreciate both how complex our calendar systems are but also how arbitrary time designations can sometimes be. In the end, I am impressed with how the calendar wheel turned out and I hope you have a deeper appreciation for our exhibit interactives!

Sources that were used to find the phases of the moon and the Hebrew calendar dates include:

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