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University of Illinois, Urbana Champaign**We aren't endorsed by this school
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ASTR 100
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Astronomy
Date
Dec 21, 2024
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15
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Chapter 1. Introduction to the SkyIntroductionAstronomy is probably the coolest science. We have the Biggest Bang, the hotteststars, the most supermassive black holes, and the darkest of matter.We invite you to come along on a series of voyages to explore the universe asastronomers understand it today. Beyond Earth are vast and magnificent realms fullof objects that have no counterpart on our home planet. Nevertheless, we hope toshow you that the evolution of the universe has been directly responsible for yourpresence on Earth today.Along your journey, you will encounter:A canyon system so large that, on Earth, it would stretch from Los Angeles toWashington, DC (Figure 1.1).Figure 1.1:Mars Mosaic. This image of Mars is centered on the Valles Marineris(Mariner Valley) complex of canyons, which is as long as the United States is wide.(Credit: modification of work by NASA)A crater and other evidence on Earth that tell us that the dinosaurs (and manyother creatures) died because of a cosmic collision.A tiny moon whose gravity is so weak that one good throw from its surfacecould put a baseball into orbit.A collapsed star so dense that to duplicate its interior we would have tosqueeze every human being on Earth into a single raindrop.Exploding stars whose violent end could wipe clean all of the life-forms on aplanet orbiting a neighboring star (Figure 1.2).
Figure1.2:Stellar Corpse. We observe the remains of a star that was seen to explode in ourskies in 1054 (and was, briefly, bright enough to be visible during the daytime).Today, the remnant is called the Crab Nebula and its central region is seen here.Such exploding stars are crucial to the development of life in the universe. (Credit:NASA, ESA, J. Hester (Arizona State University))A “cannibal galaxy” that has already consumed a number of its smaller galaxyneighbors and is not yet finished finding new victims.A radio echo that is the faint but unmistakable signal of the creation event forour universe.And many, many more.Such discoveries are what make astronomy such an exciting field—but you willexplore much more than just the objects in our universe and the latest discoveriesabout them. We will pay equal attention to the process by which we have come tounderstand the realms beyond Earth and the tools we use to increase thatunderstanding.We gather information about the cosmos from the messages the universe sends ourway. By the time you have finished reading this text, you will know a bit about how toread that message and how to understand what it is telling us.What is Astronomy?Astronomy is defined as the study of the objects that lie beyond our planet Earth and
the processes by which these objects interact with one another. We will see, though,that it is much more. It is also humanity’s attempt to organize what we learn into aclear history of the universe, from the instant of its birth in the Big Bang to thepresent moment. Throughout this book, we emphasize that science is a progressreport—one that changes constantly as new techniques and instruments allow us toprobe the universe more deeply.In considering the history of the universe, we will see again and again that thecosmos evolves; it changes in profound ways over long periods of time. For example,the universe made the carbon, the calcium, and the oxygen necessary to constructsomething as interesting and complicated as you. Today, many billions of years later,the universe has evolved into a more hospitable place for life. Tracing theevolutionary processes that continue to shape the universe is one of the mostimportant (and satisfying) parts of modern astronomy.Quick Tour of the UniverseWe can now take a brief introductory tour of the universe as astronomers understandit today to get acquainted with the types of objects you will encounter throughoutthe text. We begin at home with Earth, a nearly spherical planet about 13,000kilometers in diameter (Figure 1.3). It is one of my favorite planets—it’s where I keepmy stuff.Figure 1.3.The Earth seen from the Western hemisphere as viewed from space35,400 kilometers (about 22,000 miles) above Earth. Data about the land surfacefrom one satellite was combined with another satellite’s data about the clouds tocreate the image. (Credit: modification of work by R. Stockli, A. Nelson, F. Hasler,NASA/ GSFC/ NOAA/ USGS)Our nearest astronomical neighbor is Earth’s satellite, commonly called the Moon.The furthest humans have ever travelled (in the Universe) is the Moon, which is theclosest thing to Earth. Figure 1.4 shows Earth and the Moon drawn to scale on thesame diagram. Notice how small we have to make these bodies to fit them on thepage with the right scale. The Moon’s distance from Earth is about 30 times Earth’s
diameter, or approximately 384,000 kilometers, and it takes about a month for theMoon to revolve around Earth. The Moon’s diameter is 3476 kilometers, about onefourth the size of Earth.Figure 1.4.This image shows Earth and the Moon shown to scale for both size anddistance. (Credit: modification of work by NASA)Earth revolves around our star, the Sun, which is about 150 million kilometers away—approximately 400 times as far away from us as the Moon. We call the averageEarth–Sun distance an astronomical unit (au) because, in the early days ofastronomy, it was the most important measuring standard. The diameter of the Sunis about 1.5 million kilometers; Earth could fit comfortably inside one of the minoreruptions that occurs on the surface of our star. If the Sun were reduced to the sizeof a basketball, Earth would be a small apple seed about 30 meters from the ball.It takes Earth 1 year to go around the Sun; to make it around, we must travel atapproximately 110,000 kilometers per hour. (If you, like many students, still prefermiles to kilometers, you might find the following trick helpful. To convert kilometersto miles, just multiply kilometers by 0.6. Thus, 110,000 kilometers per hour becomes66,000 miles per hour.) Because gravity holds us firmly to Earth and there is noresistance to Earth’s motion in the vacuum of space, we participate in this extremelyfast-moving trip without being aware of it day to day.Earth is only one of eight planets that revolve around the Sun. These planets, alongwith their moons and swarms of smaller bodies such as dwarf planets, make up thesolar system (Figure 1.5). A planet is defined as a body of significant size that orbitsa star and does not produce its own light. (If a large body consistently produces itsown light, it is then called a star.) We’ll discuss planets later on, but they arespherical (or nearly so), in orbit around a star, and have cleared their orbit so thatthere are no other similarly sized objects in similar orbits.
Figure 1.5.The Sun, the planets, and some dwarf planets artificially arranged toshow them all drawn to scale. At left a small portion of the immense disk of the Sunis shown. The planets and dwarf-planets are drawn in two rows in their relativepositions from the Sun. The upper row shows the major planets from left to right:Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. In the lower roware drawn a few selected dwarf-planets. From left to right: Ceres (in the asteroid belt,and drawn below Mars), then Pluto, Haumea, Makemake, and Eris below and to theright of Neptune. (Credit: NASA)The Sun, the planets, and some dwarf planets are shown with their sizes drawn toscale. The orbits of the planets are much more widely separated than shown in thisdrawing. Notice the size of Earth compared to the giant planets. We are able to seethe nearby planets in our skies only because they reflect the light of our local star,the Sun. If the planets were much farther away, the tiny amount of light they reflectwould usually not be visible to us. The planets we have so far discovered orbitingother stars were found from the pull their gravity exerts on their parent stars, or fromthe light they block from their stars when they pass in front of them. We can’t seemost of these planets directly, although a few are now being imaged directly.The Sun is our local star, and all the other stars are also enormous balls of glowinggas (plus plasma which is ionized gas) that generate vast amounts of energy bynuclear reactions deep within. We will discuss the processes that cause stars to shinein more detail later. The other stars look faint only because they are so very far away.If we continue our basketball analogy, Proxima Centauri, the nearest star beyond theSun, which is 4.3 light-years away, would be almost 7000 kilometers from thebasketball (which is more than half the size of the Earth).When you look up at a star-filled sky on a clear night, all the stars visible to theunaided eye are part of a single collection of stars we call the Milky Way Galaxy, orsimply the Galaxy. (When referring to the Milky Way, we capitalize Galaxy; whentalking about other galaxies of stars, we use lowercase galaxy.) The Sun is one of
hundreds of billions of stars that make up the Galaxy; its extent, as we will see,staggers the human imagination.Our Galaxy looks like a giant disk with a small ball in the middle. If we could moveoutside our Galaxy and look down on the disk of the Milky Way from above, it wouldprobably resemble the galaxy in Figure 1.6, with its spiral structure outlined by theblue light of hot adolescent stars.From our position inside the Milky Way Galaxy, we cannot see through to its far rim(at least not with ordinary light) because the space between the stars is notcompletely empty. It contains a sparse distribution of gas (mostly the simplestelement, hydrogen) intermixed with tiny solid particles that we call interstellar dust.This gas and dust collect into enormous clouds in many places in the Galaxy,becoming the raw material for future generations of stars. Figure 1.7 shows an imageof the disk of the Galaxy as seen from our vantage point.Figure1.6.What our Galaxy might look like if we could see it from above. We can’t see ourown galaxy this way, of course, so this is another galaxy called NGC 1073.Astronomers name things weirdly. NGC stands for New General Catalog, but it wasmade in the late 1800’s, which means it is not new at all. NGC 1073 is thought tolook very much like our Milky Way. Here we see the giant wheel-shaped system witha bar of stars across its middle. (Credit: NASA, ESA)From our position inside the Milky Way Galaxy, we cannot see through to its far rim(at least not with ordinary light) because the space between the stars is notcompletely empty. It contains a sparse distribution of gas (mostly the simplestelement, hydrogen) intermixed with tiny solid particles that we call interstellar dust.This gas and dust collect into enormous clouds in many places in the Galaxy,becoming the raw material for future generations of stars. Figure 1.7 shows an image
of the disk of the Galaxy as seen from our vantage point.Figure 1.7.Because we are inside the Milky Way Galaxy, we see its disk in cross-section flung across the sky like a great milky white avenue of stars with dark “rifts”of dust. In this dramatic image, part of the Milky Way is seen above Trona Pinnaclesin the California desert. (Credit: Ian Norman)Typically, the interstellar material is so extremely sparse that the space betweenstars is a much better vacuum than anything we can produce in terrestriallaboratories. Yet, the dust in space, building up over thousands of light-years, canblock the light of more distant stars. Luckily, astronomers have found that stars andraw material shine with various forms of light, some of which do penetrate thedistance, and so we have been able to develop a pretty good map of the Galaxy.Recent observations, however, have also revealed a rather surprising and disturbingfact. There appears to be more—much more—to the Galaxy than meets the eye (orthe telescope). From various investigations, we have evidence that much of ourGalaxy is made of material we cannot currently observe directly with our
instruments. We call this component of the Galaxy "dark matter", as it does not emitlight (and is therefore dark). We know the dark matter is there by the pull its gravityexerts on the stars and raw material we can observe, but what this dark matter ismade of and how much of it exists remain a mystery. Furthermore, this dark matteris not confined to our Galaxy; it appears to be an important part of other stargroupings as well. We’ll talk about dark matter more later.By the way, not all stars live by themselves, as the Sun does. Many are born indouble or triple systems with two, three, or more stars revolving about each other.Because the stars influence each other in such close systems, multiple stars allow usto measure characteristics that we cannot discern from observing single stars. In anumber of places, enough stars have formed together that we recognized them asstar clusters (Figure 1.8). Some of the largest of the star clusters that astronomershave cataloged contain hundreds of thousands of stars and take up volumes of spacehundreds of light-years across.Figure1.8.This large star cluster is known by its catalog number, M9. It contains some250,000 stars and is seen more clearly from space using the Hubble SpaceTelescope. It is located roughly 25,000 light-years away. (Credit: NASA, ESA).So, we discussed planets, stars, and our Galaxy, but we also already hinted thatthere are many galaxies. In a very rough sense, you could think of the solar systemas your house or apartment and the Galaxy as your town, made up of many houses
and buildings. In the twentieth century, astronomers were able to show that, just asour world is made up of many, many towns, so the universe is made up of enormousnumbers of galaxies. (We define the universe to be everything that exists that isaccessible to our observations.) Galaxies stretch as far into space as our telescopescan see, many billions of them within the reach of modern instruments. When theywere first discovered, some astronomers called galaxies island universes, and theterm is aptly descriptive; galaxies do look like islands of stars in the vast, dark seasof intergalactic space.Our ViewOur view of the Universe has been limited (until recently) by what we can see in theSky. Our ancestors knew the patterns of the sky because they did not have TV or theinternet or smartphones and there was not much else to do at night. Theyintrinsically understood the motions of the sky and how it related to the seasons andsome even figured out how to observe the sky with the naked eye and predicteclipses. Even though their astronomy was very good, usually their world-view of theUniverse did not match what we know today.When we see the Sun rise in the East and set in the West, we know that the Sun isnot moving around our Sky, but rather that our planet, Earth, is rotating. In fact, wequickly realized that the Sun, the Moon, the planets, the stars, the Milky Way, everynight seem to rise in the East and set in the West because our Planet rotates thatway. The difference between the Earth rotating and the the Sun moving is not clearfrom the simple observation of a sunrise, but using our minds and improving oureyes with instruments provide a true understanding of the Universe. However, in thiscourse, you will learn what we still don’t know everything. There are stillfundamental questions in our quest for understanding the Universe.In this class, open your mind and try to imagine how your ancestors saw the nightsky and then imagine how your descendants will see the same night sky in hundredsof years from now. In a way, the stars connect us all. Later, we will learn that the ironin our blood was forged in the center of a star, which means that we are not onlyconnected to other humans, but we are connected to the entire cosmos too.The SeasonsOne of the fundamental facts of life at Earth’s mid-latitudes, i.e., Illinois, is that thereare significant variations in the heat we receive from the Sun during the course ofthe year. We thus divide the year into seasons, each with its different amount ofsunlight. The difference between seasons gets more pronounced the farther north orsouth from the equator we travel, and the seasons in the Southern Hemisphere arethe opposite of what we find on the northern half of Earth. With these observed factsin mind, let us ask what causes the seasons.Many people have believed that the seasons were the result of the changingdistance between Earth and the Sun. This sounds reasonable at first: it should becolder when Earth is farther from the Sun. But the facts don’t bear out thishypothesis. Although Earth’s orbit around the Sun is an ellipse, its distance from theSun varies by only about 3%. That’s not enough to cause significant variations in theSun’s heating. To make matters worse for people in North America who hold this
hypothesis, Earth is actually closest to the Sun in January, when the NorthernHemisphere is in the middle of winter. And if distance were the governing factor, whywould the two hemispheres have opposite seasons? As we shall show, the seasonsare actually caused by the 23.5° tilt of Earth’s axis.The Seasons and SunshineFigure 1.9 shows Earth’s annual path around the Sun, with Earth’s axis tilted by23.5°. Note that our axis continues to point the same direction in the sky throughoutthe year—for the North Celestial Sphere that point is the star Polaris. As Earth travelsaround the Sun, in June the Northern Hemisphere “leans into” the Sun and is moredirectly illuminated. In December, the situation is reversed: the SouthernHemisphere leans into the Sun, and the Northern Hemisphere leans away. InSeptember and March, Earth leans “sideways”—neither into the Sun nor away from it—so the two hemispheres are equally favored with sunshine.Figure 1.9.The Earth at different seasons as it circles the Sun. In June, the NorthernHemisphere “leans into” the Sun, and those in the North experience summer andhave longer days. In December, during winter in the Northern Hemisphere, theSouthern Hemisphere “leans into” the Sun and is illuminated more directly. In springand autumn, the two hemispheres receive more equal shares of sunlight. (Note thatthe dates indicated for the solstices and equinoxes are approximate; depending onthe year, they may occur a day or two earlier or later.) (Credit: OpenStax)How does the Sun’s favoring one hemisphere translate into making it warmer for usdown on the surface of Earth? There are two effects we need to consider. When welean into the Sun, sunlight hits us at a more direct angle and is more effective atheating Earth’s surface (Figure 1.10). You can get a similar effect by shining aflashlight onto a wall. If you shine the flashlight straight on, you get an intense spotof light on the wall. But if you hold the flashlight at an angle (if the wall “leans out”of the beam), then the spot of light is more spread out. Like the straight-on light, thesunlight in June is more direct and intense in the Northern Hemisphere, and hencemore effective at heating.
Figure 1.10.(a) In summer, the Sun appears high in the sky and its rays hit Earthmore directly, spreading out less. (b) In winter, the Sun is low in the sky and its raysspread out over a much wider area, becoming less effective at heating the ground.Thus, one square meter of sunlight falls on over twice the surface area in winter vs.summer. The Earth is less warmed by the Sun in winter. (Credit: OpenStax)The second effect has to do with the length of time the Sun spends above thehorizon (Figure 1.11). Even if you’ve never thought about astronomy before, we’resure you have observed that the hours of daylight increase in summer and decreasein winter. Let’s see why this happens.Figure 1.11.On June 21, the Sun rises north of east and sets north of west. Forobservers in the Northern Hemisphere of Earth, the Sun spends about 15 hoursabove the horizon in the United States, meaning more hours of daylight. OnDecember 21, the Sun rises south of east and sets south of west. It spends 9 hoursabove the horizon in the United States, which means fewer hours of daylight andmore hours of night in northern lands (and a strong need for people to holdcelebrations to cheer themselves up). On March 21 and September 21, the Sun
spends equal amounts of time above and below the horizon in both hemispheres.(Credit: OpenStax)Why is the time different? This is due to the pesky tilt of the Earth. If the Earth’s tiltwas 0° relative to the celestial equator, it would be in the sky equal daylight andnighttime every day. But the tilt causes the Sun in the sky changes as the year wearson.In June, the Sun is north of the celestial equator and spends more time with thosewho live in the Northern Hemisphere. It rises high in the sky and is above the horizonin Urbana for more than 15 hours. Thus, the Sun not only heats us with more directrays, but it also has more time to do it each day. (Notice in Figure 1.11 that theNorthern Hemisphere’s gain is the Southern Hemisphere’s loss. There the June Sun islow in the sky, meaning fewer daylight hours. In Chile, for example, June is a colder,darker time of year.) In December, when the Sun is south of the celestial equator, thesituation is reversed.Let’s look at what the Sun’s illumination on Earth looks like at some specific dates ofthe year, when these effects are at their maximum. On or about June 21 (the date wewho live in the Northern Hemisphere call the summer solstice or sometimes the firstday of summer), the Sun shines down most directly upon the Northern Hemisphereof Earth. It appears about 23° north of the equator, and thus, on that date, it passesthrough the zenith of places on Earth that are at 23° N latitude. The situation isshown in detail in Figure 1.12. To a person at 23° N (near Hawaii, for example), theSun is directly overhead at noon. This latitude, where the Sun can appear at thezenith at noon on the first day of summer, is called the Tropic of Cancer.Figure1.12.The Earth on June 21. This is the date of the summer solstice in the NorthernHemisphere. Note that as Earth turns on its axis (the line connecting the North andSouth Poles), the North Pole is in constant sunlight while the South Pole is veiled in24 hours of darkness. The Sun is at the zenith for observers on the Tropic of Cancer.
(Credit: OpenStax)We also see in Figure 1.12 that the Sun’s rays shine down all around the North Poleat the solstice. As Earth turns on its axis, the North Pole is continuously illuminatedby the Sun; all places within 23° of the pole have sunshine for 24 hours. The Sun isas far north on this date as it can get; thus, 90° – 23° (or 67° N) is the southernmostlatitude where the Sun can be seen for a full 24-hour period (sometimes called the“land of the midnight Sun”). That circle of latitude is called the Arctic Circle.Many early cultures scheduled special events around the summer solstice tocelebrate the longest days and thank their gods for making the weather warm. Thisrequired people to keep track of the lengths of the days and the northward trek ofthe Sun in order to know the right day for the “party.” (You can do the same thing bywatching for several weeks, from the same observation point, where the Sun rises orsets relative to a fixed landmark. In spring, the Sun will rise farther and farther northof east, and set farther and farther north of west, reaching the maximum around thesummer solstice.)Now look at the South Pole in Figure 1.12. On June 21, all places within 23° of theSouth Pole—that is, south of what we call the Antarctic Circle—do not see the Sun atall for 24 hours. Dr Andrew Nadolski, an Illinois graduate alumni, wintered over in2017 to work on the South Pole Telescope. In Figure 1.13, you can see the winteringover crew, who did not see the Sun for 6 months (although it is only really dark, vs.twilight, for about 11 weeks).Figure 1.13.Image of the winter over crew from 2017/2018 at the South Pole tocoordinate scientific experiences. This is the beginning of the night as you can seethe glow to the left and the Milky Way dominating the sky. Illinois graduate alumni,Dr Andrew Nadolski, spent the winter working on a new camera for the South PoleTelescope. (Credit: Hunter Davis and Matthew Smith)
The situation is reversed 6 months later, about December 21 (the date of the wintersolstice, or the first day of winter in the Northern Hemisphere), as shown in Figure1.14. Now it is the Arctic Circle that has the 24-hour night and the Antarctic Circlethat has the midnight Sun. At latitude 23° S, called the Tropic of Capricorn, the Sunpasses through the zenith at noon. Days are longer in the Southern Hemisphere andshorter in the north. In Urbana, there is only 9.25 hours of sunshine during the day. Itis winter in the Northern Hemisphere and summer in the Southern Hemisphere.Figure1.14.Earth on December 21. This is the date of the winter solstice in the NorthernHemisphere. Now the North Pole is in darkness for 24 hours and the South Pole isilluminated. The Sun is at the zenith for observers on the Tropic of Capricorn and thusis low in the sky for the residents of the Northern Hemisphere. (Credit: OpenStax)Many cultures that developed some distance north of the equator have a celebrationaround December 21 to help people deal with the depressing lack of sunlight and theoften dangerously cold temperatures. Originally, this was often a time for huddlingwith family and friends, for sharing the reserves of food and drink, and for ritualsasking the gods to return the light and heat and turn the cycle of the seasonsaround. Many cultures constructed elaborate devices for anticipating when theshortest day of the year was coming. Stonehenge in England, built long before theinvention of writing, is probably one such device. In our own time, we continue thewinter solstice tradition with various holiday celebrations around that Decemberdate.Halfway between the solstices, on about March 21 and September 21, the Sun is onthe celestial equator. From Earth, it appears above our planet’s equator and favorsneither hemisphere. Every place on Earth then receives roughly 12 hours of sunshineand 12 hours of night. The points where the Sun crosses the celestial equator arecalled the vernal (spring) and autumnal (fall) equinoxes. If the Earth had no tilt, itwould always be equinox on Earth.
The Seasons at Different LatitudesThe seasonal effects are different at different latitudes on Earth. Near the equator,for instance, all seasons are much the same. Every day of the year, the Sun is up halfthe time, so there are approximately 12 hours of sunshine and 12 hours of night.Local residents define the seasons by the amount of rain (wet season and dryseason) rather than by the amount of sunlight. As we travel north or south, theseasons become more pronounced, until we reach extreme cases in the Arctic andAntarctic.At the North Pole, all celestial objects that are north of the celestial equator arealways above the horizon and, as Earth turns, circle around parallel to it. The Sun isnorth of the celestial equator from about March 21 to September 21, so at the NorthPole, the Sun rises when it reaches the vernal equinox and sets when it reaches theautumnal equinox. Each year there are 6 months where you can see the Sun at eachpole, followed by 6 months where you can not see the Sun.Clarifications about the Real WorldMost of the discussions described the rising and setting of the Sun and stars as theywould appear if Earth had little or no atmosphere. In reality, however, theatmosphere has the curious effect of allowing us to see a little way “over thehorizon.” This effect is a result of refraction, the bending of light passing through airor water. Because of this atmospheric refraction (and the fact that the Sun is not apoint of light but a disk), the Sun appears to rise earlier and to set later than it wouldif no atmosphere were present.In addition, the atmosphere scatters light and provides some twilight illuminationeven when the Sun is below the horizon. Astronomers define morning twilight asbeginning when the Sun is 18° below the horizon, and evening twilight extends untilthe Sun sinks more than 18° below the horizon.These atmospheric effects require small corrections in many of our statements aboutthe seasons. At the equinoxes, for example, the Sun appears to be above the horizonfor a few minutes longer than 12 hours, and below the horizon for fewer than 12hours. These effects are most dramatic at Earth’s poles, where the Sun actually canbe seen more than a week before it reaches the celestial equator.You probably know that the summer solstice (June 21) is not the warmest day of theyear, even if it is the longest. The hottest months in the Northern Hemisphere areJuly and August. This is because our weather involves the air and water coveringEarth’s surface, and these large reservoirs do not heat up instantaneously. You haveprobably observed this effect for yourself; for example, a pond does not get warmthe moment the Sun rises but is warmest late in the afternoon, after it has had timeto absorb the Sun’s heat. In the same way, Earth gets warmer after it has had achance to absorb the extra sunlight that is the Sun’s summer gift to us. And thecoldest times of winter are a month or more after the winter solstice.