Where on earth is Aries?

[StuM]

Originally Posted by goboatingnow

Yes but the distances are so great and the timescales so great that the differences are minimal , the GHA of Aries is re computed regularly and hence accommodates these changes.

ALL the celestial coordinates (relative positions of stars and Aries) are recalcuated periodically (the effective date is known as an epoch). The "current" position of Aries is actually based on observations and calculations as at the year 2000 (Epoch J2000). Before 2000, the position used was based on 1950. (Epoch B1950).

Stars such as Barnard's Star need to have their position recalculated more frequently -Barnard's Star changes position by about 10" per year!

[SeanPatrick]

Originally Posted by Wideocean7

SeanPatrick - impressive looking website. I'll be checking that out.

Thank you! If you scroll down you will find an animation which illustrates this very topic.

Originally Posted by waterman46

If you look at "GHA Aires" in the almanac it can be seen that Aires rotates about 15 deg per hour, same as the sun.

Well, almost the same. Aries appears to make one revolution about every 23h 56m 04s. This is known as "sidereal time" and is the reason for "right ascension" being given in H:M:S. That way, if you know the sidereal time at your location, you will know when specific bodies are overhead - which is rather useful if you want to see them through a telescope. In fact, the body will be transiting your meridian at the sidereal time which matches the body's R.A., which is the best time to view it through a telescope.

Incidentally, the term "right ascension" refers to the hour angle of the part of the celestial equator which is rising with the body when the celestial equator is perpendicular [at a right angle] to the horizon (i.e. when the observer is at the Equator). A body rising when the observer is at any other latitude is said to have an "oblique ascension". The hour angle of the part of the celestial equator which is rising in that case will not match the hour angle of the body. But all of that is entirely irrelevant and useless for celestial navigation.

[Wideocean7]

Originally Posted by Ninedotsix

We have in recent times been provided evidence of expansion of the universe, so doesn't it mean that all bodies are moving apart from one another, at increasing speed? ~ 9.6

For what it's worth, I understand that every single star we use for navigation (or for that matter, that we can see with the naked eye) is within the Milky Way galaxy. So I am guessing that it's the expansion of the Milky Way that matters to celestial nav, not how the broader Universe is expanding.

[StuM]

Originally Posted by Ninedotsix

We have in recent times been provided evidence of expansion of the universe, so doesn't it mean that all bodies are moving apart from one another, at increasing speed? I feel sure it is a small amount, over lifetimes, but it is at least hypothesized that the rate of expansion will at some point in time, if it continues to be reckoned, equal the speed of light?

Maybe not.

https://www.pnas.org/doi/full/10.1073/pnas.2200539119

[Adelie]

The prime meridian is 0* longitude on the earth. Is is an arbitrarily chosen N-S line on the earth. There were long term arguments about where it should be. For a long time the French had it running thru Paris, the English thru Greenwich and various other countries thru locations of their choice. Over time the French and English came to dominate internationally until an international conference settled on Greenwich which pissed off the French.

For the sky/celestial sphere first point of Aries is an arbitrary point in the sky where celestial longitude counts from. Celestial longitude is called SHA (sidereal hour angle and measured in degrees, arc-minutes & arcs-econds. ) for navigation and right ascension (measured in hours, minutes & seconds) for astronomical work.

When the first maps of the sky were made the first point of Aries was define by a physical event. But that physical event drifts so they would have to change the SHA for every star every year or two.

GHA (Greenwich Hour Angle) is the earth longitude of the point on the earth directly under the celestial body.

GHA-Aries is the longitude on the earth directly under the First point of Aries at any given second.

The reason we track the first point of Aries is because it is computationally easier to keep track of that one point and make adjustments to get the GHA for each of the stars. When you look the almanac for each day (actually 2 pages covering 3 days). The is a column of data for Aries and a list of offsets for each of the 57 navigational stars. The alternative would be to have 57 columns, 1 for each star.

So if you need GHA-Sirius you add GHA-Aries + SHA-Sirius = GHA-Sirius

[Adelie]

Originally Posted by Ninedotsix

Thanks for your message.

I think I understand, but my reasoning is conflicted: We have in recent times been provided evidence of expansion of the universe, so doesn't it mean that all bodies are moving apart from one another, at increasing speed? I feel sure it is a small amount, over lifetimes, but it is at least hypothesized that the rate of expansion will at some point in time, if it continues to be reckoned, equal the speed of light? This last is just a separate point to my initial meaning in this post, which is - aren't all objects changing position, so the effect on systems of measure must take this into account? Perhaps a bit esoteric, practically speaking.....

~ 9.6

Current cosmological thinking is that that the universe is expanding, not just that the things in it are getting further apart but space itself is getting bigger.

Things in your vicinity are moving relative to you but not necessarily away from you.

Think of it like being at an ice rink or roller rink. All the other folks there are moving relative to you but on average are not getting closer or further away.
But a roller rink in the Americas is getting further away from another rink in Europe or Africa due to the expansion of the Atlantic due to continental drift. The effect is on the order of mm/yr. Not noticeable except to precision instruments specifically looking for the effect.

The expansion of the universe is similarly distant and unimportant to practical celestial navigation except on a HUGELY vaster scale