How does the lowest visible declination determine which constellations are visible from a location?

Answer 1

Constellation located south of the most southern visible declination cannot be seen.

Answer 2

The lowest visible declination sets a southern limit on the stars (and hence, the constellations) that are visible from any given location, because they would never rise above the horizon from a given location. If Earth were airless, the lowest visible declination would be Ds = L - 90 where L is the observer's latitude. Any star or constellation with a declination below this limit is permanently below the horizon and not visible. Because the air interferes with observation, especially near the horizon, the actual physical limit is a little above this theoretical number.

Answer 3

Declination is the amount north or south of the celestial equator (a projection of our equator on the stars). Any constellation below the visible declination is below the horizon and therefore cannot be observed. This declination is determined by how far north or south of the earth's equator an observer is located.

Answer 4

If a Constellation is below the lowest visible declination then it is below the horizon or at least so low in the sky that the atmosphere is too thick to see the Consellation clearly.

Answer 5

Imagine you are at the N pole. If you look up the Declination is 90 and if you look down the lowest stars you can possibly see are at the equator, which is Dec 0. If you go half way to the equator you are at Dec +45. Dec +45 is what you see looking straight up. Now you can see N to +90 and S in past the equator to Dec -45. If you go to the equator and look straight up you'll see Dec 0. Looking North your horizon will be at Dec +45 and looking South it will be at Dec -45. Keep going to -45 and you'll be able to see Dec 0 at one horizon and Dec -95, the S pole, at the other, and now at the S pole you can see only N as far as the equator.

What it means is that when standing on a big ball, the Earth, you can never see more than 90 degrees in one direction, and never more than 180 degrees in all directions. Everywhere else is hidden from you.

In real life trees, mountains, hills, and other such prevent us from seeing a complete stretch of sky, so we don't get the theoretical maximums.

So where you are determines what constellations you see.

The other thing that determines what constellations you see is the season: that is, where we are in the Earth's journey around the sun. Night time is always "the side that is away from the sun." That is, night time is when the Earth beneath out feet blocks the sun, literally keeping us in shadow.

But in addition to having a 24 hour rotation, we also go around the sun once a year. In June we are on one side of the sun and in January we are on the opposite side. So night time in June shows you one side of the universe when we are in the shadow, and night time in January shows the other side of the universe when we are in the shadow. In Autumn and Spring we are half way in between. So what constellations we see marks the passage of time.

When we say the "sun enters a constellation" we are referring to the fact that as we make our orbit around the sun, we see it against different parts of the sky. In July we can't see Orion because to see Orion we have to look up in the sky in the daytime, and the sun is in the way.

Hope that helps....