@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 Observe the Sun's Face Safely by Projection
March 18th 1605
Last updated December 8th 2025
For too long, we have been told the heavens are perfect and unchanging. This is a falsehood! The Sun itself, that glorious lamp, has blemishes upon its face. To look directly upon it is to invite blindness, a fate I would not wish on any honest seeker of truth. Therefore, I present a method I have perfected to project the Sun's image safely onto a surface using a perspicillum, or telescope. By this technique, any person can witness these 'maculae' for themselves and track their stately passage, proving with their own eyes that the Sun turns upon its own axis. Observe, measure, and know the truth!
You will need:
A perspicillum (or 'telescope'), even a simple one of modest power will suffice.
A flat, white card or smooth sheet of paper to serve as your projection screen.
A sturdy stand or mounting to hold your instrument steady, for a wavering image is useless for measurement.
A darkened chamber, or a large piece of card to shield your screen from stray light.
A fine-pointed stylus or piece of charcoal for marking your observations.
A clock or timepiece to ensure observations are made at consistent intervals.
1. A Grave Warning: Preserve Your Sight!
I must begin with a command of utmost gravity: YOU MUST NEVER look at the Sun directly through your perspicillum. To do so is to invite instant and permanent blindness. The instrument gathers the Sun's light to a terrible, burning point. Our work is done with shadows and projections only. Heed this, or you will see nothing ever again.
2. Arrange Your Apparatus in Shadow
Set your instrument upon its stand with a clear view of the sun. Position your white screen a foot or two behind the eyepiece. If you are not in a dark room, use a large card with a hole cut for the eyepiece to cast a deep shadow upon your screen. Contrast is the mother of clarity.
3. Aiming by Shadow, Not by Sight
To aim your perspicillum, do not look through it. Instead, observe its shadow on the ground. Adjust the tube until its shadow is at its smallest and most perfectly circular. At that moment, the instrument is aimed squarely at the Sun, and a bright disc of light should appear upon your screen.
4. Achieving a Sharp Focus
Now, adjust the eyepiece by sliding it in or out. You will see the circle of light on your screen shrink and grow, its edges blurring and sharpening. Your task is to find the position where the edge of the Sun's disc is most crisp and well-defined. A sharp image is essential for seeing the finer maculae.
5. Witness the Sun's Imperfections
Gaze upon the projected disc. You will see it is not a perfect orb of light as the ancients claimed. Look for small, dark blemishes. These are the sunspots! Note their size, their shape, and their position. You are seeing the true face of the Sun, a thing of change and activity.
6. Record Your Observations with Diligence
With your stylus, carefully trace the circumference of the projected solar disc onto your paper. Then, with great precision, mark the exact location of every spot you can discern. Note the date and the hour of your observation upon the same sheet. A scientist's greatest tool is a meticulous record.
7. Prove the Sun's Rotation Through Time
Return the next day at the very same hour and repeat your observation on a new sheet. You will discover the spots have moved! By tracking their path across the disc from day to day, you provide irrefutable, mathematical proof that the Sun itself rotates upon an axis, a magnificent discovery accessible to any diligent observer.
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