The Stars Through The Seasons (1970)

[Music] as the seasons change on earth the noon position of the sun seems to change at noon on the first day of summer the sun reaches its highest point and then as the months pass the sun is lower in the sky at noon each day on the first day of winter the sun is at its lowest noon position the sun's position changes with each season and so do the positions of the stars we see as the earth carries us around the sun during the year we look into slightly different parts of space each night we can't see the stars in the daytime but after the sun has set the stars appear exactly which stars you see and where you see them will depend on three things where you are what time it is and what day of the year it is we'll see why let's think about just one of the stars the north star we know that the north star is almost directly over the earth's north pole light rays from the north star spread out in all directions but it is so far from earth that all the rays that reach us are coming from nearly exactly the same direction they're nearly parallel the distance to every star is very great no matter where earth is in its orbit light rays that reach earth from any star are always nearly parallel now think of people at different parts of the world to someone at the north pole light rays from the north star always seem to come from directly overhead he turns slowly under it this makes all the stars he can see appear to turn around the north star they are all circumpolar stars they never rise or set now think of a person in the middle latitudes of the northern hemisphere and think again of the nearly parallel light rays from the north star if we take his point of view by tilting the camera you can see that the north star appears lower in the sky not directly overhead and as the rotating earth is moving him under the stars it seems to him that the stars are moving during the night only the few stars close to the north star remain above the horizon these are the circumpolar stars the other stars he sees rise and set like the sun now picture someone on the equator to him all the stars but one the north star seem to rise and set the north star rests exactly on the northern horizon people living in the middle latitudes of the southern hemisphere can't see the north star their southern sky has no pole star it has the southern cross a constellation which is nearly directly over the south pole now let's return to the middle latitudes of the northern hemisphere if we look to the south we can follow a constellation on one of its nightly journeys across the sky canis major the big dog is above the horizon for only about seven hours its position changes during that time so where you see it in the sky or whether it will be above the horizon at all depends on what time it is so the position of the stars depends on where you are on the earth and what time it is now let's see how it also depends on the day of the year let's picture ourselves on earth at four different times during the year a night in spring summer autumn and then winter now earth is in space and there are stars in all directions but we can only see the stars in the part of space that is away from the sun the part we face at night our view slowly changes as we orbit the sun so the constellations we see in summer are not the same as those we see in winter this is because a star rises about four minutes earlier each night than it did the night before we can see why by thinking of two ways of measuring the length of a day we usually think of a day as the time it takes for the earth to make one complete rotation in relation to the sun this kind of day is called a solar day and as you know there are about 365 and a quarter solar days in a year however astronomers also measure a different kind of day the time it takes the earth to make one complete rotation in relation to a distant star from the rising of a star on one night to its rising on the next night is called a sidereal day there are about 366 and a quarter sidereal days in one year let's see why there is this one day difference between a sidereal day and a solar day picture the earth and the sun we'll point an arrow from the earth to the sun and another one to a star here are the nearly parallel light rays from a star now to represent a solar day we'll rotate the earth until the arrow that pointed to the sun points to it again suppose this is the first day of spring march 21st we'll notice the positions of both these arrows on the same day each month during the spring here is the earth on april 21st the bottom arrow is pointing directly at the sun and the earth has turned through exactly 31 solar days since march 21st but notice the other arrow it too has been turning but it had made 31 turns when it was in this position so at the moment when the earth finishes its 31st solar day it is already part way into its 32nd sidereal day keep watching the position of the upper arrow in relation to the star by the first day of summer it is a quarter of a turn away from the star by the first day of autumn we've had half a day more of sidereal days than of solar days and by the time the year is over we've made one full turn more in relation to a distant star than we have in relation to the sun this difference of one extra day when divided among the three hundred sixty five and a quarter days of our year comes out to about four minutes the sidereal day is about four minutes longer than the solar day because of this time difference circumpolar constellations like the big dipper have slightly different positions at the same time each night they seem to start their circles through the sky a little ahead of where they started the night before a distance they would move in about four minutes as the earth rotates any star in the northern or southern sky that is not circumpolar will appear to rise or set about four minutes earlier each night so for instance suppose we're looking south on the first day of winter serious the brightest star in the sky rises at about 10 pm and sets at about 5 a.m throughout the winter we can see sirius in the sky in the early evening but by the first day of spring sirius is beginning to rise in the afternoon and we can't see it it's only visible after the sun sets and then we see it for just a few hours before it sets then through the summer sirius is in the sky only during the daytime and we cannot see it at all in the summer the sun and sirius are in the same part of the sky and at night we're turned in the opposite direction in space by autumn sirius has begun rising early in the morning a few hours before dawn but the steady change of about four minutes per day which is about two hours per month continues by the first day of winter sirius is rising around 10 pm the same as it did on the first day of winter last year and it is visible throughout the night because the best time of year to see sirius is winter sirius is called a winter star stars and constellations are often grouped according to the seasons of the year when they are easiest to see so now let's look at the stars through the seasons what would we see on the first day of spring in the northern sky at about eight in the evening the big dipper is above and east of the north star in the part of space that is to our south the stars of the constellation leo are prominent the appearance of leo the lion announces the coming of spring to the north in the part of space that we face on summer nights the big dipper is high in the sky at about 8 pm in the southern sky there are three bright stars in three constellations deneb in the northern cross vega in lyra and altair in the eagle they form an easy to find triangle in the summer sky to the north in the part of space that we face on autumn nights we see the big dipper at eight in the evening below and west of the north star in the southern sky the stars that make up the great square of pegasus the flying horse are prominent and then on nights when we look north into space from our winter position we see the big dipper near the horizon at about eight in the evening and in the south the constellation orion the hunter dominates the southern sky and announces the coming of winter and so the pattern of the stars you see slowly changes this happens because you're an inhabitant of a spherical planet that rotates on its axis as it revolves about its central star the sun turning you at night toward different regions of space through the seasons of the year you