# Star map - finding stars

Assume you are given a star map that doesn't have the name of the stars on it. You know the latitude, longitude of the observer and the local time (year, month, day, local time). How could I proceed to find the stars, planets?

Notes: This requirement is given by a problem with the photo shown above. You cannot go outside, nor use other resources. The stellar map represents the sky of a city ($44^\circ 26'$ N, $26^\circ 06'$ E, local time 00:47, date 04.03.2016).

1. Mark on the map the cardinal points
2. Identify on the map the planets from the solar system
3. Identify Polaris
4. Identy the circumpolarity zone. Identify at least 2 constellations outside the zone
5. Find the galactic equator and the ecliptic
• What coordinates don't you know? You can find the stars by going outside and looking up, but you don't need a map to do that. Please clarify. – James K Dec 23 '17 at 22:51
• Those 3 questions should be in the question rather than a comment. – Pere Dec 24 '17 at 0:58
• @AlexS the comment above is correct. Comments can be used to help to clarify issues with questions, but they might be deleted at some point in the future. All clarifications should be made to the original post above by editing it. Thanks! – uhoh Dec 26 '17 at 2:03
• Can we see fig 1 or an approximation thereof? Remember, questions on the Olympiad my deliberately include extraneous information not required to solve the problem. – barrycarter Dec 26 '17 at 3:07
• @AlexS Bravo! Looks much nicer after you've added all of the additional information; thanks! – uhoh Dec 26 '17 at 17:32

NOTE: Information on question and comments has been changing a lot, so please beware that some parts of the answer may be related to previous versions of the question.

This is a question to check if you know the sky. Once you can identify constellations and planets on the sky, the answer on the map is quite obvious.

To answer this question you need to be familiar with the northern hemisphere sky. If you don't know it, you would need a properly labelled map to answer the question.

Suggested steps:

• Assuming that the map just represents the visible portion of sky and the horizon is printed in it, first step would be to look for Polaris. The point of the horizon which is closest to Polaris is the North. The South is in the opposite edge of the map. To locate East and West, place the map with South lower and then East is at left and West is at right - just opposite as they would be in a map of Earth, but this is a map of Sky.

• I think it would be easier to identify constellations before planets. The easier constellations are likely to be the circumpolar ones (those that we can always see in the northern hemisphere).

• To find the planets, you should find the Ecliptic by looking at zodiacal constellations and then check for points that don't belong to those constellations. Of course, to do that you need to know the constellations. A helpful piece of information is that at 00:47 local time, the Sun, Mercury and Venus can't be seen. If the map shows the planets visible with naked eye, Mars, Jupiter and Saturn may be there in addition to the Moon.

Your map is drawn inside of a circle. That circle represents the horizon, and Polaris is not in the centre of the map. Therefore, there is a point in the horizon that is the closest point to Polaris. That point is the north.

All this just comes from the fact that Polaris is always in the north. If you look at Polaris in the sky, you are facing north - just as if you look at the Sun at noon, you are facing south.

To better understand this, I suggest taking a sky map and a compass, going outside and trying to find Polaris tonight in the actual sky - tonight or the next available starry night.

Edit: finding Polaris

To find a constellation or a given star, you need to know the constellations or compare your map of the actual sky with a labelled star map. The question is not very different to (for example) "find Ireland in a blank world map": either you know the world and where Ireland is, or you have a labelled map to compare. Star maps aren't that different from geographical maps. In Earth maps you know where you are by looking at geographical features, and in sky you know where you are by looking at stars positions. To find Polaris, look for Ursa Minor; finding Ursa Major or Casiopea may help, too.

Addition after the map was included in the question:

North is in the right side of the map. Polaris is the dot halfway from the center and the right side.

The easiest clue to find Polaris is the conspicuous Ursa Major in the center of the map. As others have suggested, two stars of the Ursa Major point to Polaris. Furthermore, some stars from Ursa Minor are easy to spot over Polaris.

• And how do I know where North is? On the map, Polaris is just a small dot. As you know, there are many other stars brighter than that. Also how do I know that at 00:47, at a random time, a random place those planets can't be seen? – Alex S Dec 24 '17 at 10:43
• Your map is drawn inside of a circle, isn't it? That circle represents the horizon, and Polaris is not in the centre of the map. Therefore, there is a point in the horizon that is the closest point to Polaris. That point is the north. – Pere Dec 24 '17 at 10:55
• And how do I find Polaris? There are other stars or planets which are brighter than Polaris. I get your point. Since the latitude is about $45^\circ$ N, it should be somewhere in the middle between the center which is the zenith, and the North. But the problem is that I don't know in what direction is this North point – Alex S Dec 24 '17 at 11:16
• You need to know the constellations or compare your map of the actual sky with a labelled star map. The question is not very different to (for example) "find Ireland in a blank world map": either you know the world and where Ireland is, or you have a labelled map to compare. The sky maps aren't that different from geographical maps. In Earth maps you know where you are by looking at geographical features, and in sky you know where you are by looking at stars position. To find Polaris, look for Ursa Minor; finding Ursa Major or Casiopea may help, too. – Pere Dec 24 '17 at 11:32
• I don't think you understand that the North on the map I am given is not necessarily on the top of the map! It could be also on the bottom of the map, which you would normally have associated with South, the map can be rotated anyhow! – Alex S Dec 24 '17 at 19:51

I think the previous hints were already quite extensive, so I'll comment on the planets.
You should know that Venus and Mercury can't be visible at midnight (which is not a random time for those planets, think why!).
The total number of possibly visible planets in the whole solar system is 5. If you're taking an Olympiad on the topic then I'd assume you had an introductory class into astronomy at least, then it should be clear why it's 5.

You have been given the information that there are 3 planets visible. So from 5-2=3 you know which planets that must be. Then the only thing you have to do is to order them according to magnitude to assign them their Names.

• I have not been given the information that there are 3 visible planets. – Alex S Dec 24 '17 at 19:41
• Where have I ever said that? – Alex S Dec 24 '17 at 19:41
• @AlexS: Ah, I've misread. That was question 3 of your part. Then I guess you'll have to estimate from the magnitudes, if they're given. – AtmosphericPrisonEscape Dec 24 '17 at 20:04
• You are also not given the magnitudes – Alex S Dec 24 '17 at 20:34

Interesting question :). To find polaris, find Ursa Major, then use the pointer stars (the two in the bowl of the big dipper furthest away from the "handle" part) and use those to find Ursa Minor/Polaris. [Edit - since you're at around 45 degrees latitude, Polaris is going to be somewhere on a circle centred on the middle of the map with radius going about halfway to the horizon, which gives you a cross check]. That gives you two constellations. The W shape of Casseopeia is also circumpolar - roughly the same distance opposite polaris from the big dipper star where the "bowl" joins the "handle". That's three. Depending on the time of year, you should be able to find at least two more easily recognisable constellations - Orion and Leo are easy to remember, as are Lyra and Delphinus and Sagitta.

To find north, draw a line through Polaris from the center of the map. Where that hits the horizon is North. Going the opposite way is South. Rotate the map so North is at the top and West is to the right, East to the left (remember you're looking up at the Sky, so they're reversed from a normal looking-down-at-the-ground map).

For circumpolar stars, it depends on the map projection used. If it's using a planisphere projection, then you can draw a circle centered on the polaris and just touching the horizon (But in that case, I don't think the horizon is going to be a circle unless there's something special about 45 degrees).

For the celestial equator, you want a circle 90 degrees out centred on Polaris (again, assuming a planisphere projection). That goes through the northernmost star in Orions belt, if orion is visible. Otherwise, at 45 degrees latitude, it's two thirds of the way from Polaris to the horizon on your north-south line. (North to Polaris is 45 degrees, Polaris to south is 135 degrees (45 to zenith then 45 to equator then 45 to horizon).

For the planets (Mars, Jupiter, and Saturn) either remember roughly where jupiter and saturn are (since the date is close to the present), or order by magnitude. (Mercury and Venus are ruled out since as inner planets they're closer to the sun, and near midnight they're on the the other side of the earth and not visible).

If the map isn't using a planisphere style projection (where circles of equal declination map to circles centred on polaris on the map) then trying to draw the celestial equator and circumpolar stars needs a much better memory for how the declination lines fit with the constellations, since they won't be circular on the map