Stellarium – The Seasons

Name: ___________________________ Date: ____________________________ The Seasons (Stellarium Exercise #6) Earth is the planet we know best. It is also the most complex planet in the Solar System. Oceans, volcanoes, earthquakes, weather, erosion, heat, cold… the Earth has it all! The main reason the Earth is so complex (apart from the interference of human beings!) is its proximity to the Sun. The Earth is just warm enough to keep water in a liquid form on its surface. A little further from the Sun, and all the water would freeze. A little closer to the Sun, and all the water would boil away. The Earth is perfectly balanced between these two extremes. This remarkable coincidence accounts for almost all of the Earth’s attributes. The Sun is the source of all the Earth’s energy, and the source, ultimately, of all the Earth’s complexity. One of the primary features of the Earth is the seasons. The seasons go through a familiar yearlong cycle, created by the relationship between the Earth and the Sun. Let’s take a look at these two bodies to try and further understand what that relationship is. PART A Many people think that the seasons are caused by the Earth’s proximity to the Sun – the Earth is closer to the Sun in Summer, and farther in Winter, or so people think. Let’s first see if this hypothesis is true by measuring the distance from the Earth to the Sun at various times of year. Start Stellarium. Use the Search function to find, select and center the Sun. Turn off the Ground and Atmosphere by pressing G and A, to make it easier to see the Sun. Information about the Sun will be in the usual place – the upper left-hand corner of the screen. One of the pieces of information about the Sun here is it’s distance from the Earth at this moment, listed as Distance, in astronomical units (au). Change the date to your birthday. • What is the distance of the Earth from the Sun on your birthday? _______________________ Enter this distance from the Sun in the Earth-Sun Distance table below (keep only 3 decimal places) for the month of your birthday. Change the date to one month later (the easiest way to do this is to press the “]” key, which moves time forward by one week, four times), and enter the new distance a month later in the table, and then repeat until you’ve measured the distance to the Sun at least once in each month. Earth-Sun Distance Month Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Distance (AU) • What month is the Earth closest to the Sun? _____________________________________ • What month is the Earth farthest from the Sun? __________________________________ • Are seasons caused by the distance of the Earth to the Sun? ___________________ 3/19/2024 1 • How do your measurements prove or disprove this hypothesis? _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ Now let’s look at the Earth, as seen from the Sun, over the course of the year. Make sure that the Ground, Atmosphere and Fog are all turned off. Go to the Sun by selecting it (click on it or find it using the Search window) and pressing CTRL-G. Then look at the Earth by finding it via the Search window. Change the Date & Time to March 21 at 12 Noon (the year doesn’t matter) and press the 7 key to stop time. Click on the Switch Between Equatorial and Azimuthal Mount button in the toolbar to keep the plane of the Earth’s orbit horizontal (an easier way to do this is simply to press CTRL-M). Open the Sky and Viewing Options window and, in the Planets and Satellites section, put checks next to Show Solar System Onjects, Show Planet Markers and Show Planet Orbits. Close the Sky and Viewing Options window. The red line you see is the path of the orbit of the Earth around the Sun, also known as the Ecliptic. Now zoom in until the Earth fills the screen. Make sure the Earth is selected – click on it until the red “crosshairs” appear around it. Look carefully for the Earth’s North and South Poles on the planet. Picture an imaginary line running through the center of the Earth from the North Pole to the South Pole. • Is this North-South Pole line perpendicular (at a 90° angle) to the red line of the Ecliptic? ______ • Estimate the angle between the N-S Pole line and the line perpendicular to the Ecliptic ________ This angle is called the Earth’s obliquity – the angle between the Ecliptic plane and the Earth’s equatorial plane. This angle is what causes the seasons! Let’s see how. Look carefully at where L.A. is on the Earth right now on your screen, and then look at the center of the Earth on screen – the point where the Sun is shining directly down on the Earth. This point, where the Sun is shining straight down on the Earth, is called the Sub-Solar Point. It marks the spot on Earth where, if you were standing there, the Sun would be directly overhead. • Is the Sun shining straight down on L.A. On March 21? (Remember, you are looking at the Earth from the point of view of the Sun – so think of the Sun as being right behind your head as you look at the Earth) ________________________________ • Where on Earth is the sub-Solar point on March 21? In other words, where is the Sun shining straight down on March 21: above the Equator, on the Equator, or below the Equator? ________. Change the date to June 21 – the first day of summer in Los Angeles. Look carefully at the continents on Earth. The Earth should look like its shifted a bit. Change the date back and forth between March 21 and June 21 if you don’t see it right away. With the date set at June 21, again, remember that the Sun is right behind your head… • Is the Sub Solar Point above the Equator, on the Equator, or below the Equator on June 21? ________ • Here in L.A., on June 21, is the Sun shining more directly down on us, less directly down, or at the same angle, as compared to March 21? _________________________________________ 3/19/2024 2 This day, June 21, is the first day of summer (in the Northern Hemisphere), also known as the Summer Solstice. Now change the Date to September 21. • Is the Sub Solar Point above the Equator, on the Equator, or below the Equator on this day?_____ • Here in L.A., is the Sun shining more directly down on us, less directly down, or at the same angle on September 21, as compared to March 21? __________________________________ This day, September 21, is the first day of Autumn, also known as the Autumnal Equinox. Change the Date to December 21. • Is the Sub Solar Point above the Equator, on the Equator, or below the Equator on this day? ____ • Here in L.A., is the Sun shining more directly down on us, less directly down, or at the same angle on December 21, as compared to March 21? ___________________________________ This day, December 21, is the first day of Winter, also known as the Winter Solstice. Let’s try to understand why this time of year is so cold – at least in the Northern Hemisphere. • If you wanted to stay warm, would you hold a heat lamp directly over your head, or hold it off to your side it at a low angle? _____________________________________________________________________________ • So why is it colder in Winter than in Summer? ________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ So part of the reason we have seasons is the angle of the sunlight shining on the Earth at different times of year. A higher angle means more direct sunlight, and hence warmer – that’s called summer. A lower angle means less direct sunlight, and hence colder – that’s called winter. But there’s more to the story than that! PART B Let’s see how all this looks from the Earth. Return to L.A. by clicking on the Earth and pressing CTRL-G. Turn the Ground back on by pressing G and turn the atmosphere on by pressing A. Search for and center the Sun. Open the View window and un-check the Show Planet Orbits box. Close the View window. Set the time and date to March 21 (the year doesn’t matter), at 4:30 AM local time. Now zoom out and drag the sky around until you can see the whole sky and North is at the top of the screen. Press L a few times to speed up time. You should see the sky lighten as dawn approaches. Stop the motion (the 7 key) just as the Sun rises. If it’s hard to see exactly where the Sun is, turn off the atmosphere by pressing the A key. 3/19/2024 3 • What time is sunrise on March 21? _______________________________________ • What is the Azimuth of the Sun as it rises? (Remember, if you click on the Sun, data on the Sun will be displayed) RECORD ONLY THE DEGREES IN ALL MEASUREMENTS OF ALTITUDES AND AZIMUTHS IN THIS EXERCISE – IGNORE ANY FRACTIONS OF A DEGREE, i.e., ANY MINUTES AND SECONDS ____________ Press L a few times again and watch as the Sun rises and crosses the sky. Stop the motion at 12 Noon. Again, look at the information on the Sun, which should still be displayed on screen in the upper left-hand corner. • What is the altitude of the Sun at noon on March 21? ________________________ • What is our latitude here in Los Angeles? _____________________________________ • Subtract this last answer from 90º, and what do you get? _________________________ • How does this answer compare to the altitude of the Sun at Noon on March 21? _______ Press L a few times again and watch the Sun move toward sunset. Stop the motion when the Sun is just setting. • What time is sunset on March 21? _________________________________________ • How many hours of daylight are there on March 21 (subtract the Sun’s rising time from the setting time)? ________________________________________ Now set the date to June 21 (The Summer Solstice). Again, set the time to 4:30 AM. Press L a few times and watch the Sun rise. Stop the motion when the Sun is just rising. • What time is sunrise on June 21? ___________________________________________ • What is the azimuth of the Sun as it rises on June 21? __________________________ • Is the Sun rising farther North or South on June 21, compared to March 21? _________ Press L a few times to make time pass quickly, and watch the Sun move through the sky. Stop the motion at 12 Noon. • What is the altitude of the Sun at noon on June 21? ____________________________ • Is the Sun higher, lower, or at the same position in the sky at noon on June 21, compared to March 21? __________________________________________________________ Press L a few times again to let time pass quickly, and stop the motion when the Sun is just setting (i.e., is on the Western horizon). • What time is sunset on June 21? ___________________________________________ 3/19/2024 4 • How many hours of daylight are there on June 21? _____________________________ So not only is the Sun shining more directly down on us in the Summer (it’s closer to the zenith), but the Sun is up for a longer period of time! No wonder Summers are hotter! Set the date to September 21 – the Autumnal Equinox – and repeat the process. • What is the time of sunrise on September 21 (the Autumnal Equinox)? ______________ • What is the azimuth of sunrise on September 21 (the Autumnal Equinox)? __________ • What is the altitude of the Sun at Noon on the Autumnal Equinox? ________________ • How many hours of daylight are there on the Autumnal Equinox? __________________ Now set the date to December 21 – the Winter Solstice – and repeat the process. • What is the time of sunrise on December 21 (the Winter Solstice)? _________________ • What is the azimuth of sunrise on December 21 (the Winter Solstice)? _____________ • What is the altitude of the Sun at Noon on the Winter Solstice? ___________________ • How many hours of daylight are there on the Winter Solstice? ____________________ Now you can see why Winters are cold – at least in the Northern Hemisphere! What about in the Southern Hemisphere? Buenos Aires, Argentina is as far South of the Equator as Los Angeles is North. Let’s see what the Winter and Summer Solstices are like there. Open the Location window, and type Buenos Aires into the Location search box. Click on Buenos Aires, Argentina in the list that appears to change your location to that city. • What is the Latitude and Longitude of Buenos Aires? Lat: ____________ Lon: _____________ Close the Location window. NOTE: Even when you’re observing from Buenos Aires, your time will still be set to Los Angeles time, so the listed rising and setting times for the Sun in Buenos Aires will seem weird! Just record the rising and setting times, no matter how strange they seem, and subtract one from the other to answer the following questions. • How many hours of daylight are there in Buenos Aires on June 21? ________________________ • How many hours of daylight in Buenos Aires on December 21? ________________________ • Compare these answers with the answers to the same questions for L.A. on the same days. What do you notice? __________________________________________________________ 3/19/2024 5 What about at the North Pole? What are seasons like there? Let’s see! Change the location to the North Pole by re-opening the Location window and clicking on the upper edge of the map, as close to the edge as possible, to change the Latitude to 90° North. Re-set the date to March 21. Set the time to 3 AM. • Where’s the Sun? _______________________________________________________ Press L a few times and watch the hours go by. • What happens to the Sun? ________________________________________________ Change the date to June 21. Set the time to 3 AM. • Where’s the Sun? _______________________________________________________ Let the hours pass. • What happens to the Sun? _______________________________________________ • How many hours of sunlight are there on June 21 at the North Pole? ________________ • What’s the highest altitude the Sun ever gets in the sky at the North Pole? ___________ Set the date to December 21. Set the time to 12 Noon. • Where’s the Sun? _________________________________________________________ • How many hours of sunlight are there on December 21 at the North Pole? ____________ The North Pole is a very strange place! And very cold! Now you know why! Finally, what are the Solstices like on the Equator? Let’s find out! Change your location to Quito, Ecuador by typing Quito into the Search box in the Location window and then clicking on Quito, Ecuador. Quito is very close to being right on the Equator. Set the date to June 21. Remember, the time will still be set to L.A. time, so the rising and setting times for the Sun will seem weird! That’s okay – just use the difference between the rising and setting times to answer the questions below. • How many hours of sunlight at the Equator on June 21? ________________________ • How many hours of sunlight at the Equator on December 21? ___________________ • What would you expect seasons to be like on the Equator? Why? ______________________________________________________________________________ ______________________________________________________________________________ 3/19/2024 6 As you can see, the Seasons are caused by the relative position of the Earth’s surface and the Sun. The Earth’s axis is pointed in a fixed direction is space. As the Earth revolves around the Sun, that direction appears to swing around the sky from the point of view of the Sun. That exposes the Earth to Sunlight at differing angles and for differing times, depending on your location on Earth. Write a brief conclusion on the next page, explaining what you learned in this assignment. _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ 3/19/2024 7

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