# Tag Info

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### On Mars, why are the seasons "strongly amplified" in the southern hemisphere and masked in the northern hemisphere?

Your solution is correct. Mars has a perihelion that is, coincidentally, quite close to the southern Hemisphere summer solstice. Perihelion is actually about one (Earth) month before the solstice. ...
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### On Mars, why are the seasons "strongly amplified" in the southern hemisphere and masked in the northern hemisphere?

Apart from the eccentricity you pointed out that has a role in a difference in the weather, one more factors that plays a role is the difference in the terrain and topology between the hemisphere. ...
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### Straightforward derivation of the sunrise equation

We can start by converting between equatorial coordinates (right ascension $\alpha$ and declination $\delta$) to horizontal coordinates (azimuth $\text{Az}$ and elevation/altitude $a$). If you want to ...
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### When was/will Polaris's declination be maximised?

Soon after 2100: In 2018 Polaris was 0.66° away from the pole of rotation (1.4 times the Moon disc) and so revolves around the pole in a small circle 1.3° in diameter. It will be closest to the pole (...
• 87.4k
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### Measuring misalignment between two positions on sky

Assuming you mean the angle between the meridian line through A and the great circle that goes through points A and B, then it goes something like this. Define vectors from the origin to A and B ...
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### A Noob's question on declination

I don't think this particularly useful way to think about it. The equator is the equator, Cancer is 23.4 North, and Capricorn is 23.4 South. The equator is a great circle on the sphere of the sky, but ...
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### What is the area of the Summer Triangle?

The area of a triangle enclosed by 3 stars on the celestial sphere, in square degrees, is given by: $$A = \frac{180}{\pi}\times E$$ Where E is the spherical excess and is equal to the sum of all ...
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### Is the Ra/Dec of Alnitak in Orion's Belt known to be correct?

I tried to reproduce your result. I took the values of RA and DEC from Wikipedia: Alnitak, Alnilam, Mintaka. The values are the following: Alnitak: Ra = 05h 40m 45.52666s, Dec = −01° 56′ 34.2649″ ...
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### Deriving the apparent angular speed of a star

As you mention, the angular speed of the celestial sphere on the equator is 0.25° per minute. The speed on a declination circle is proportional to the circumference of that circle, relative to the ...
• 8,862
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### Which is the shape of the sky?

For computer software, the easiest way to take a sphere (and/or hemisphere) and flatten it into a flat shape (usually a rectangle) is the equi-rectangular projection (also known as the plate carrée), ...
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### How to manually locate a certain declination in the sky for a given location? (no sextant available)

Jacob's staff is probably all you need. It is a pole with a cross arm. By positioning the arm at a certain point on the pole and sighting down the pole one can get a position on the sky. A skilled ...
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### What is meant by *topocentric* right ascension and declination?

You are correct that topocentric coordinates are for the position of "close" objects, corrected for observing from the Earth's surface instead of the theoretical center. The topocentric and ...
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### At what time of the day of autumnal equinox the Sun's equatorial coordinates reach max value?

As noted in comments, at the september equinox, the sun's declination is decreasing (slightly) during the day, so the declination will be slightly higher in the morning than in the evening. But the ...
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### How to calculate the nodal precession range of the Moon with respect to the celestial equator?

You want to know the range of RA where the Moon's orbital plane crosses the celestial equator from south to north, so we need to find the line of intersection of those two planes. The vector ...
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### an exercise from Spherical Astronomy by Smart

Constant for every star with different declination's. Though you would have to assume the the declination is such that it can have an azimuth of 90 each, not all stars will. Depending on your ...
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### Where is 0 R.A.?

0 RA is the direction from the center of the Earth to the vernal equinox. You've already imagined the plane in space created by projecting the equator into space. Now consider the plane formed by the ...
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### Centre (projected) of a galaxy cluster

Short answer, no, they don't. Longer answer, it's complicated. There are, in essence, five different measures for the centre of a galaxy cluster, based on different physical properties of the cluster,...
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### Why are tides biggest during the equinoxes?

Only a syzygial tide during/near an equinox is the strongest. This means, there must be either new or full moon. In general, syzygial tides are strong because three bodies (Earth, Moon, Sun) align ...
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### Need Simple equation for Rise, Transit, and Set time

The calculations are not trivial, but they are encapsulated in software libraries such as pyephem, which has examples of finding rise, set and transit If you want to understand how these are ...
• 87.4k
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### how steady over time are right ascension and declination values?

The values are more steady for objects close to Ecliptic poles, but in genenral, they change by a few arcseconds every year. Yes, we lack an absolutely fixed frame of reference.
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### Moon equatorial coordinates

It's a bit late but hopefully it will help others in the future. The calculations can be found on this site, article from Keith Burnett (keith@xylem.demon.co.uk) The first problem is that you use ...
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### How to calculate declination of Moon?

The complexity of the formula depends on the precision you need. We can make a crude approximation with two sine waves: one for the Moon's travel eastward around the ecliptic over a tropical month \...
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### How to calculate the ground track of the Moon's position on the Earth's surface?

Given a date and time, the position of the Moon can be calculated to provide the declination and right ascension. The sub-point of the Moon (the point on the Earth at which the Moon is at the zenith) ...
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### Help with Converting Right Ascension and Declination into 3D Vectors

The stars should fit $r = \sqrt{x^2 + y^2 + z^2}$ but seem to be plotted in the $x = r$ plane instead. Conventionally the x axis is at (α=0h, δ=0°), the y axis is at (α=6h, &...
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### What is the rate of change by day for a given right ascension?

Answer to your question RA changes just a little bit through time (because of minor effects like parallax), but we can just say it is constant for a specific non-Sun star. Same applies to declination. ...
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Interaction of the Planes The orbital plane of the Moon maintains a relatively constant inclination to the ecliptic, which currently is about $5^{\large\circ}09'$, and the nodes precess along the ...