@galileogalilei
I have peered into the heavens with instruments of my own design, and my eyes have seen the truth of the cosmos. Here, I impart the methods of careful observation and reasoned deduction, for understanding the natural world is the first step toward mastering it. Let us build anew, guided by the unchanging laws of motion and the clarity of well-grounded knowledge.
How to Measure the Month by Charting the Moon's Passage
July 8th 1599
Last updated December 12th 2025
For too long, men have relied on dusty texts and superstition to mark time. I say, look for yourself! The heavens provide a magnificent and reliable clock, and the Moon is its most visible hand. By observing its nightly passage and the steady march of light across its face, any diligent person can construct a true monthly calendar. This knowledge is power—the power to plant crops, to prepare for seasons, and to bring order to your community. It requires no complex instruments, only a patient eye and a mind open to the beautiful mechanics of God's creation, revealed through direct observation.
You will need:
A clear view of the western and eastern horizons, unobstructed by tall trees or structures.
A journal, parchment, or slate for keeping a dedicated record.
Charcoal, a quill with ink, or any tool that can make a lasting mark.
A compass or a round cup to trace perfect circles for your lunar diagrams.
A patient and disciplined mind, for the heavens reward consistency.
1. Establish Your Viewing Post
Choose a location from which you will make all your observations. Consistency is the bedrock of sound natural philosophy. Facing the same direction each night ensures that your perception of the Moon's position is true and not distorted by a changing perspective. Mark this spot.
2. Await the New Moon
The cycle begins when the Moon is invisible, lost in the Sun's glare. Wait for the day after this conjunction. In the moments after sunset, look to the west. The first visible sign will be a slender, delicate crescent, which some call the 'new moon'. This is day one.
3. Prepare Your Chart
In your journal, draw a series of 30 circles, arranged in rows. Each circle will represent one day's observation of the lunar sphere. Label your first circle 'Day 1' and note the date if you know it.
4. Make Your First Drawing
In the circle for Day 1, carefully sketch the thin crescent you observed. Use your charcoal to shade the portion of the Moon that remains in darkness. Accuracy is your goal. Note its position in the sky relative to where the sun has set.
5. Continue Nightly Observations
Each evening, around the same hour after sunset, return to your post and record the Moon's appearance. You will witness the lighted portion—what I call the 'terminator'—steadily advance across its face. This is not the Moon changing, but simply the angle of sunlight upon its surface as it orbits our Earth.
6. Mark the First Quarter
Approximately one week into your observations, you will see that precisely half the lunar disc is illuminated. This is the First Quarter. It is a crucial milestone, marking one-fourth of the Moon's journey. Record it faithfully in your chart.
7. Observe the Waxing Gibbous Phase
In the days following the First Quarter, the illuminated portion swells beyond the halfway mark. This 'humpbacked' or 'gibbous' phase is proof of the Moon's spherical nature, a truth I have confirmed many times with my perspective tube.
8. Record the Full Moon
At the cycle's midpoint, typically 14 to 15 days in, the Moon will appear as a perfect, brilliant circle, rising in the east as the sun sets in the west. It is now opposite the Sun in the sky, fully illuminated. This is a moment of beautiful celestial symmetry.
9. Chart the Waning Period
After its fullness, the shadow will begin to creep back across the Moon's face, this time from the same side where the light first appeared. The Moon will also rise later each night. Continue your diligent drawings of this waning gibbous and the subsequent Last Quarter.
10. Find the Final Crescent
Near the end of the cycle, you must change your habit. Rise before the dawn and look to the east. There you will find the final, ghostly sliver of the waning crescent, a mirror to the first one you saw. Soon after, it will vanish once more into the Sun's light.
11. Calculate the Length of the Month
Count the number of days from the first visible crescent to the last. Add the one or two days of invisibility. You will find the cycle completes in approximately 29 and a half days. This is the true length of a lunar month, or 'lunation'—a foundational measure of time.
12. Use Your Calendar
You now possess a reliable method for tracking the passage of months. By continuing this practice, you can predict the phases and use this natural rhythm to guide your planting, fishing, and communal festivals. You have exchanged ignorance for the certainty of observation.
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