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I'm not an astronomer, please excuse my non-formal language.

Since we are located in one of the arms of the Milky way, the center of the galaxy should be in one direction from our location, while in the other direction, we stare into the "other side".

So at nights as we are in the orbit area that is between the sun and the center of the galaxy we are able to see the center (the "milky-way"), while when in the other side of the orbit, at nights, we are not able to see it.

Does this mean we only see the milky way (meaning the milky-way's famous pattern) half of the year?

In the below diagram, the small circle is our year-orbit around our sun. Red dot is the earth location on day 0 of the year and the green dot is the earth location on day 180th of the year.

I know that as we are orbiting our sun, it also orbits the galaxy, but since it is so slow, it is negligible for this questions.

diagram

Is this reasonable?

I mean, if we look at the sky, we will see the center of the galaxy only during half of the year?

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No, although there are times when it can't be seen, it isn't true that it is visible for 183 days of the year.

The general question could be "If I take an arbitrary location on the sky (say a randomly chosen star) will it be visible for half the year? The answer is "it depends on the star!" Polaris and many other stars are circumpolar for Northern hemisphere observers and are visible every night, contrariwise, Polaris is never visible for Southern Hemisphere observers.

Now, the galactic centre happens to be located in Sagittarius, so co-incidentally is quite close to the plane of the solar system. This means that for about two months of the year, the sun is too close to make observations easy, certainly in visible light from the Earth's surface. At other times it will be visible, perhaps briefly. Sagittarius and the galactic centre are also quite a few degrees South of the equator, so it is more easily visible from the southern hemisphere.

At 50 degrees North, The galactic centre sets at twilight at the start of October and doesn't rise until dawn at the middle of February, between February and October (about 7½ months) the galactic centre is above the horizon at some point during the night. (The exact number of days depends on your definition of "dawn" and "above the horizon") More southerly observers will see the galactic centre for longer. 50 degrees South, the galactic centre is above the horizon at some time during the night between January and November. In Antarctica, the galactic centre is circumpolar, and so is above the horizon every night. But as the summer sun in Antarctica doesn't set, there are days when you can't "see" it. The optimum location would be 30 degrees South (or in space)

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  • $\begingroup$ Yes of course! +1 $\endgroup$ – planetmaker Jul 15 at 18:46
  • $\begingroup$ Note also that even if the galactic centre was in the plane of the ecliptic, like the questioner expected, "some point during the night" is an important qualification. An object doesn't necessarily need to be as much as 90 degrees away from the Sun in order to be visible in the night sky for some period of time after the Sun sets or before the Sun rises. Venus, for example, would never be "in the night sky" according to the questioner's diagram, since it's always on the Sun's side of any line you draw through the Earth perpendicular to the Sun-Earth axis. $\endgroup$ – Steve Jessop Jul 15 at 19:45
  • $\begingroup$ Basically what the questioner expects to be "in the night sky" is really, "above the horizon, at a latitude in the tropics that you choose to match the time of year, at midnight": that is to say in the half of the ecliptic that's on the non-Sunny side of that perpendicular line. But it can be plenty dark at times other than midnight, to see objects that set before midnight or rise after midnight! $\endgroup$ – Steve Jessop Jul 15 at 19:54
  • $\begingroup$ Sagittarius has two T and one G, not the other way around. $\endgroup$ – hobbs Jul 15 at 20:12
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    $\begingroup$ @JamesK: I think you might have to fix that yourself. Stackexchange deems correct spelling to be insufficiently important to allow anyone other than the author to make such a small change to an answer. $\endgroup$ – Steve Jessop Jul 15 at 20:35
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To simplify, I used Starry Night to generate a visualization.

enter image description here

The yellow arc is a piece of the ecliptic. You can see (in faint gray) the center of the yellow arc is labeled 'Dec'. The Sun will be in this part of the sky in December.

You can also see planets of the inner solar system and in the background, part of Sagittarius (and the Teapot asterism) is labeled. Apologies for using Sgr A East instead of Sgr A* ... (basically the same spot at this scale).

So the answer to your question is, if we assume you are standing on the Earth and have to contend with an atmosphere, then yes ... you can only view the area near the center of the galaxy for part of the year. If you didn't have to deal with an atmosphere then (e.g. if you were floating in space) then you could see it all the time -- you wouldn't have to deal with the bright blue sky blocking out your view to all the stars in that part of the sky.

The solar system's plane isn't perfectly edge-on to the center of the galaxy ... it is is slightly south of the ecliptic plane. And... you can certainly see it more than just "half" of the year. As long as it is dark and clear and the Sun isn't too close to that part of the ecliptic than you can see. There's little hope of that in the month of December and even November or January might be a little tough. But certainly February through October it is visible at some time during the night.

For example, in February you would see it in the pre-dawn hours. In October you would see it in the early evening after sunset. But that part of the sky is visible for easily 3/4 of the year.

There are exceptions. If you're at the North Pole... you never see it at any time of the year. At the South Pole it is a circumpolar object which never sets. But the Sun is up for roughly half the year and down for the other half so you can only see it during the dark half of the year.

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    $\begingroup$ I never realized Van Gogh went into that level of detail in Starry Night ;D $\endgroup$ – Davy M Jul 15 at 20:22

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