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I'm doing a science olympiad event called Reach for the Stars, and one question that I've encountered provides a star chart and time of day, and you must figure out what month of the year it is from. The strategy that I developed includes figuring out which direction is west (usually a star chart from the night and the Sun sets in the west), then going in that direction estimating which zodiac constellation in Sun is in (and in turn approximately what month it is).

However, I realize that this method would not work if it was taken late at night, much after the Sun sets, because although I am fairly sure that each hour the sky rotates about 15 degrees, I can't estimate what constellation the Sun would be in.

Is there a more definite way to figure this out?

Edit: The time in the example chart is 8:30 PM in the time zone of the observer

Example star chart: enter image description here

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The night sky is the hemisphere of the sky which is pointed away from the Sun.

I note that the Eath orbits the Sun.

It takes one year to orbit around the sun.

So today one hemisphere of the sky will pointed toward the Sun, and it will be imposible to see stars in the daylight in that hemisphere, while the opposite hemisphere of the sky will be pointed away from the Sun and with will be possible to see stars in the night time in that hemisphere.

And 6 months from now the Earth will be on the opposite side of the Sun. Thus the hemisphere which once faceed toward the Sun will now face away from the Sun and it will be possible to see stars in the night time in that hemisphere, while the hemisphere of the sky which used to face away from the Sun will now face toward the Sun and it will be impossible to see stars in the day time in that hemisphere.

As the Earth orbits the Sun, the stars visible at night slowly change, until after 6 months a whole different half of the sky is visible at night.

So you need to recognize some of the stars and constellations in the sky chart. Stars and constellations near the edges of the sky chart will be low on the horizon at that time and date. Stars and constellations in the center of the chart will be near the zenith of the sky at that time and date. Stars and constellations about halfway between the center and the edge will be about 45 degress above the horizon.

If stars at one edge of the charat will be at the eastern horizon at that time and date, stars on the opposite side of the chart will be at the western horizon at that time and date. If stars at one edge of the chart are at the norther horizon at that time and date, stars at the opposite edge of the chart will be at the southern horizon at that time and date.

So you need to recognize as many of the constellations in the sky chart as you can. I am not very good a recognizing constellations but I do recognize a few.

You need to figure out what the dotted line represents. The celestial equator, the ecliptic, or the galactic plane?

And you might want to figure out how ancient astronomers figured out which constellations the Sun was in at a specific date, and even it's coordinates, if they couldn't see any stars near the Sun during daylight. They must have done something very clever.

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The way the star chart is drawn, the points labeled 1 and 3 are on the celestial equator, the 0 degree declination reference. The tip-off is that a line between them passes through Orion, the top-right star in Orion's belt, which is very close to the celestial equator. (Orion is the constellation labeled B.) North is, therefore, the point labeled 2. The constellation appropriately labeled W is Casseopeia. The star furthest right in Casseopeia is very close to the vernal equinox, also known as the first point of Aries, which is the right ascension 0 degree reference. Those two references are both obvious if you look at an online star chart. You'll have to wade through some astrology to find the one you need. You're looking for a chart with a grid. That should be all you need to solve the problem. It does have a unique solution. Considering that the Olympiad is a competition, I'm sure that the rules prohibit getting the answer from a consultant. Even if you're not competing, keep in mind this is a public forum.

The dotted line has nothing to do with this specific problem. There may be other questions about the chart that relate to it.

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First determine the North (South) direction by drawing a line from the center through Polaris (Sigma Octantis). The other principal directions follow from that.

You need to obtain the coordinates of a well-recognized star in equatorial coordinates (right ascension and declination), not too near the celestial pole. Given your chart, it's no problem as it shows the sky for a location near or even exactly at the equator. As it's a question from a competition I will only sketch the path briefly: The stellar coordinates allows you to calculate the local hour angle of it and from that the sidereal time. You then have to find the date where the sidereal time matches your given mean solar time. Depending on accuracy and possibly further details given in the task, you will also have to consider the equation of time.

OLD ANSWER where the time was not yet given:

A star chart is not unique, it's ambiguous for date vs. time. Just as you state: it could be from 22h today. Or it could be the sky at 21h in 14 days or 20h in a month. Generally every half month you have to remove one hour from the clock and it is identically valid.

Obviously a star chart is of limited use for times where the Sun is over the horizon or for the early twilight time where you still don't see any stars.

So what you can do is figure out for what date / time combinations the star chart is valid. In year books usually several of those are given for the star charts printed. Assuming that people usually observe in the early evening hours, one can get an estimate for the intended month though (stating the observation time).

Thus practically I'd use a programme like Stellarium and try find a month where for the chosen observation hour the night sky looks like shown. Mind also it is WAY more accurate to find North (South) direction in a star chart by drawing a line from the centre through Polaris (Sigma Octantis). All other directions follow.

You can break ambiguity of time and date, if your chart shows solar system objects like planets which change their position with respect to the stars.

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    $\begingroup$ The question mentions that the time of day is provided. $\endgroup$
    – notovny
    May 5 at 16:08
  • $\begingroup$ Thanks, that's an edit made 9 hours after I wrote my answer... $\endgroup$ May 6 at 6:41

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