![]() ![]() The idea was simply to use Jupiter in place of Venus to transfer the solar right ascension. Jupiter would not have been available to Tycho's eyes in the daytime, but I found that my instrument could image it. Sun->Jupiter->Star: With Venus marginally useless, I turned to Jupiter. The blemishes on the solar disk are almost all dust particles on my pellicle, not sunspots. Venus is the small dot near the center of the reticle. Here is a screenshot from one of my attempts to measure the separation angle between the Sun and Venus. I also found that my measurements of the Venus-to-Sun separation angles were noisy and rendered the right ascension transfer unfeasible. I was really only able to get one evening's useful measurements of Antares (alphSco) relative to Venus. When the Sun had set far enough for the brighter stars to become detectable by my instrument, Venus was already at a very low altitude, typically below 20 degrees, and sinking fast. I could only observe Venus when it was in the evening western sky (my eastern horizon is blocked by trees) but its evening apparition in 2021 was not particularly advantageous. I decided to try Tycho's methods, but they proved difficult for me due to my poor horizons. These formulas, formatted as tables of numbers, provided a solar 'ephemeris', but its usefulness depended on the knowledge of time and Tycho's clocks (the best available in 1580) still had poor accuracy. Also, the position of the Sun had been studied for millenia before Tycho, so formulas that gave the Sun's position accurately were available to him. See the second illustration on this page. The Sun's right ascension can be found on any clear day through the measurement of the Sun's meridian altitude. This allowed Tycho to measure the right ascension of Venus relative to the Sun during the day, and then use that same right ascension (adjusted for the slow orbital motion of Venus) to measure the position of bright stars, thus transferring the right ascension of the Sun to the nighttime stars. And, Venus can often be seen in the morning or evening sky when some stars are still visible. Sun->Venus->Star: Although the stars are not visible during the daytime, the planet Venus can be seen by the unaided eye (telescopes had not been invented yet in Tycho's time) even while the Sun is up. When the stars appear at night, the Sun is gone, so how to use it as a reference? Tycho found an ingenious solution. It was only later found out that this point moves slowly across the sky over the centuries, a process known as precession.Īll this was fine, but it creates one essential observational problem: the right ascension zero point is defined by the position of the Sun, but the stars cannot be observed during the day. This coordinate became known as the 'right ascension' and the reference point, known as the 'First point of Aries', was where right ascension was zero. The definition of that longitudinal reference point was the point in the sky where the Sun was located at the spring equinox. The problem was solved by letting the Sun, and the Earth's orbital motion, determine a spot on the sky that could be used for the stellar 'longitude' reference point. In the perpendicular direction, the direction of rotational movement, there is no natural absolute reference point and one is back to picking some star as the reference point in that coordinate ('longitude'). This coordinate became known as declination, defined as 90 degrees minus the star's distance from the pole. ![]() Because the stars, at night, seemed to rotate about the pole, a coordinate that reflects a star's distance from the pole, a sort of 'latitude', gives that coordinate an 'absolute' reference point. It is possible to pick a star, say Vega, and assign it the coordinates (zero, zero), but this leads to a number of problems such as how to compare positions to other catalogs that used a different method.Ī more natural way was to recognize that the rotation of the earth suggests a set of coordinates related to the apparent fixed north rotational pole of the sky. The stars can be measured relative to each other, but then one can only produce a catalog of relative positions. There was a simple but profound problem in the early days of astronomy, which was to establish the 'absolute' position of the stars in the sky. ![]()
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