This is where I put some lightly edited versions of my notes. Most of my longform notes follow the source material very closely; I do not put those here. Occasionally, though, some of my notes feel more like my own thoughts on a topic rather than direct notes, and I have put some snippets of these on this page. I hold no obligation to these being an exact reflection of the notes I have in my Obsidian vault.
Note that within my vault they would link to other detailed notes (that are heavily based on the source material and, therefore, not included here) so some context is necessarily missing. I am also using markdown and not bothering to change that to e.g. italics or bold. Please keep in mind these are snippets, often taken from a set of notes showing a chain of thought, and they may not make sense standing alone — they're not polished!
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On Venus, the Stars Rise Later
Original Obsidian file name: Venus' retrograde rotation, stars rise later each night
Tags: #astrophysics #physics #orbital_mechanics
Each night, the stars rise 4 minutes earlier. The Earth takes approximately 23 hours, 56 minutes and 4.09 seconds to rotate once relative to the background stars, i.e. the length of time between one star crossing the meridian, then crossing it again. However, it takes a little longer than this, approximately 24 hours, for the Sun to re-cross the meridian, because during this period the Earth has also been moving around the Sun in its orbit. Therefore, the Earth has to rotate a little extra, approximately 1º, for the Sun to be on the meridian again. It takes roughly 4 minutes for Earth to rotate 1º, hence, the stars rise 4 minutes "earlier" each day, in comparison to the Sun and our usual clocks (which measure solar time). (The stars do not experience this effect due to their great distance.)
This phenomenon—the stars rising earlier each night—applies to most planets. However, it does not apply to Venus. Venus has retrograde rotation. (If we define its orbit as anticlockwise, its rotation is clockwise.) Therefore, the Sun reaches the meridian again *before* the stars do, and the stars rise *later* each night compared to the Sun.
References and Context:
- An Introduction to Modern Astrophysics by Carroll and Ostlie
Sidereal and Synodic Time on Uranus
Original Obsidian file name: sidereal and synodic time on Uranus
Tags: #astrophysics #physics #orbital_mechanics
The stars rise later each night, like on Venus, because Uranus also rotates in retrograde. Therefore, Uranus' synodic day is shorter than its sidereal day, unlike Earth.
A significant difference between Uranus and Venus is Uranus' extreme tilt of its rotational axis. As a result, when Uranus' rotational axis is tangential to its orbit, the Sun and stars rise in roughly the same place. When Uranus' axis is normal to the tangent of its orbit, the Sun is stationary in the sky and the stars rise at 90º to it. When Uranus is halfway between these two positions, the Sun and stars rise at 45º to each other. (Draw a diagram if you need help processing it.)
References and Context:
- An Introduction to Modern Astrophysics by Carroll and Ostlie
