Unit 2: Naked-Eye Observations PDF

ASTR 1205 Unit 2 Dr. Bryan Rowsell Unit 2: Naked−Eye Observations 2.1 Patterns in the Night Sky Imagine you’re an astronomer in an ancient civilization. Why does your civilization care about astronomy? − timekeeping − navigation − religi...

ASTR 1205 Unit 2 Dr. Bryan Rowsell Unit 2: Naked−Eye Observations 2.1 Patterns in the Night Sky Imagine you’re an astronomer in an ancient civilization. Why does your civilization care about astronomy? − timekeeping − navigation − religion, culture, storytelling, teaching What are some of the things you might notice as you look at the night sky every night for many years? − some stars are brighter than others − connect the dots of brighter objects − objects move in predictable paths through the night sky Naked−Eye Observations ·2−1· 2.1 Patterns in the Night Sky ASTR 1205 Unit 2 Dr. Bryan Rowsell Constellations Ancient cultures connected the dots to make pictures in the stars. These stars are often totally unrelated to each other, i.e. they are usually much further apart than they appear on the celestial sphere. SEBASTIEN BEAUCOURT / LOOK AT SCIENCES / SCIENCE PHOTO LIBRARY Modern astronomers needed to agree on where one constellation ended and another began in order to clearly communicate the location of their discoveries. 1930: International Astronomical Union formally defined 88 constellation boundaries. Naked−Eye Observations ·2−2· 2.1 Patterns in the Night Sky ASTR 1205 Unit 2 Dr. Bryan Rowsell A constellation is: a region of the night sky, not the connect−the−dots pattern. Every star in the sky is part of a constellation. A pattern of stars that is not a constellation is called an asterism, e.g. the big dipper; it is not a constellation, but part of Ursa Major (which is a constellation) The Celestial Sphere Mentioned in Unit 1, the celestial sphere is the projection of the night sky onto an imaginary sphere surrounding the Earth. The ecliptic is the path the Sun follows as it appears to encircle the celestial sphere. In reality, it’s the Earth that’s moving… Naked−Eye Observations ·2−3· 2.1 Patterns in the Night Sky ASTR 1205 Unit 2 Dr. Bryan Rowsell The celestial equator isn’t the same as the ecliptic….why? The tilt of the Earth’s axis at 23.5°. The Milky Way A band of light encircling the celestial sphere passing through many constellations. Naked−Eye Observations ·2−4· 2.1 Patterns in the Night Sky ASTR 1205 Unit 2 Dr. Bryan Rowsell The Local Sky The celestial sphere is a great way to think about how the universe projects onto what we see from Earth, but our local sky is what we actually see from our position on Earth (latitude) which can be viewed as half a sphere, or a hemisphere. We use direction (i.e. north, south−east, etc. or degrees from North clockwise) and altitude, measured in degrees above the horizon. What is the maximum possible altitude (in degrees?) What is the maximum possible direction (in degrees)? 360° A star located due SE has what direction in degrees? approx. 120° Astronomers can’t use miles, km, etc. to measure the size of objects or the distance between them on the celestial sphere. It doesn’t make sense to say the Moon is 1 inch across or that a star is 10 m above the horizon. Angular size of an object is the angle it appears to span in your field of view (i.e. projected on the celestial sphere). A complete circle around the celestial sphere (such as at the celestial equator) equals 360 degrees. Another way to think of it is to point two sticks (or your arms) at each end of the object you want to measure, then measure the angle where the two sticks meet. Naked−Eye Observations ·2−5· 2.1 Patterns in the Night Sky ASTR 1205 Unit 2 Dr. Bryan Rowsell With your arms fully outstretched: What is the angular size of your instructors head? At the back of the room, a pinky finger might cover your instructor’s head, making the angular size to them is ______. Near the front, it might take two pinky fingers to cover, so the angular size of your instructor’s head to them is ______. If you observe the night sky for a long time, you’ll see that the universe seems to be circling around us with stars moving gradually from east to west. Early astronomers thought this meant the universe revolves around us, we now know it’s more the opposite. Naked−Eye Observations ·2−6· 2.1 Patterns in the Night Sky ASTR 1205 Unit 2 Dr. Bryan Rowsell Every object in the celestial sphere makes a daily rotation on the sky, but note we only see half the sky at any given time. The tilt of the Earth complicates this even further. Latitude determines which stars you can see rise & set, which stars are circumpolar, and which stars you never see. Stars that are perpetually above the horizon are circumpolar, i.e. in the northern hemisphere, these stars are near the North Celestial Pole. Is there anywhere on Earth where an observer wouldn’t see any circumpolar stars? (a) No, all locations on Earth have at least a few circumpolar stars (b) Yes, at the South Pole (c) Yes, most places in the Southern Hemisphere (d) Yes, at the equator. Latitude measures north−south position on the Earth (longitude measures east−west, but that’s less important now). Naked−Eye Observations ·2−7· 2.1 Patterns in the Night Sky ASTR 1205 Unit 2 Dr. Bryan Rowsell The altitude of the celestial pole is equal to your latitude! i.e. if the North Celestial Pole is 80° above the north horizon, your latitude is 80°! If the North Celestial Pole is 90° above the north horizon, where are you? If the North Celestial Pole is at the horizon, where are you? North Star (Polaris): is not special because it’s bright (there are at least 50 brighter objects in the night sky), it is useful because: it’s very very close to the northern celestial pole True or False: All stars to an observer on the North Pole are circumpolar? Not only does the night sky vary with latitude, but it also varies based on time of year. As the Earth orbits the Sun, the night side of the Earth looks out to different parts of the Milky Way Galaxy. Naked−Eye Observations ·2−8· 2.1 Patterns in the Night Sky ASTR 1205 Unit 2 Dr. Bryan Rowsell What zodiac sign (the constellations the sun passes through, i.e. the constellations on the ecliptic) should be quite visible tonight? Can we observe that same sign 6 months from now? 2.2 The Reason for the Seasons The Earth orbits in an ellipse around the Sun, so the Winter season must be when the Earth is farthest from the Sun, right? Naked−Eye Observations ·2−9· 2.2 Reason for Seasons ASTR 1205 Unit 2 Dr. Bryan Rowsell The difference between perihelion and aphelion is pretty minor (about 5%). The real reason is the axial tilt of Earth, and the amount of sunlight the sun−side of Earth receives during the seasons. This shows the Sun’s path on the solstices and equinoxes at a latitude of about 40° (about where Toronto is). Notice anything unique? The sun only rises/sets due exactly E/W on the equinoxes. We define four unique moments in the year: Naked−Eye Observations · 2 − 10 · 2.2 Reason for Seasons ASTR 1205 Unit 2 Dr. Bryan Rowsell Solstices are where each hemisphere gets the least/most amount of sunlight. In the Northern Hemisphere, which solstice results in the most amount of sunlight? summer, obviously! Equinoxes are where the hemispheres go from being tipped toward the Sun to being tipped away from the sun (or vice versa). Earth is like a spinning top. Its axis of rotation doesn’t always point in the same direction—it wobbles, or precesses. It takes about 26,000 years to complete one precession cycle, but the angle of the axis relative to vertical is always close to 23.5°. 2.3 The Moon, Our Constant Companion The Earth’s moon is only moon in the Solar System that is nameless. Naked−Eye Observations · 2 − 11 · 2.3 The Moon ASTR 1205 Unit 2 Dr. Bryan Rowsell Relative to the Earth−Sun distance, the Earth−Moon distance (0.0026 AU) so the sun appears the same from the Moon as it does from Earth. The moon’s orbit is elliptical (more so than many other moons in the solar system). Perigee is when the Moon is closest to Earth (analogous to perihelion) and Apogee is when the Moon is farthest from Earth (analogous to aphelion). Supermoon? No such thing…the “supermoon”, when the moon is full and at its perigee, is only 7% bigger than when the moon is at apogee and full. Difference between a 10” pizza and a 9” pizza. It only looks big when near the horizon! The large dark spots on the Moon are called mare (pl. maria), where impact craters have been filled in by lava from the Moon’s mantle; they appear to look like bodies of water and they are called “seas”. Naked−Eye Observations · 2 − 12 · 2.3 The Moon ASTR 1205 Unit 2 Dr. Bryan Rowsell Naked−Eye Observations · 2 − 13 · 2.3 The Moon ASTR 1205 Unit 2 Dr. Bryan Rowsell Here is a template of the Sun and Earth you can use to help you understand the moon phases. N Naked−Eye Observations · 2 − 14 · 2.3 The Moon ASTR 1205 Unit 2 Dr. Bryan Rowsell The moon orbits the Earth counter−clockwise (matching the rotation of the Earth), so why don’t we see the entire Moon over a lunar month? The Moon rotates on its axis at the same rate as it orbits the Earth, thus only one half of the Moon is visible on Earth, otherwise called synchronous rotation. The Earth−Moon orbit at approximately 5° tilt off the ecliptic. How many times in a month will the Moon cross the ecliptic? Twice! These are called nodes. Thus it is possible to have eclipses when the Earth, Moon and Sun are all in the same plane (note Earth and Sun are always in the same plane!) ECLIPTIC Naked−Eye Observations · 2 − 15 · 2.3 The Moon ASTR 1205 Unit 2 Dr. Bryan Rowsell lunar Earth lies between the Sun and Moon, Earth’s eclipse shadow falls on the Moon solar eclipse Moon lies between Sun and Earth, Moon’s shadow falls on Earth Wait…the Moon is way smaller than the Sun, how can the Moon eclipse the Sun? Go back to angular size again. What is the angular size (diameter) of the moon? approx. 0.52° What is the angular size (diameter) of the Sun? approx. 0.53° Thus the Moon, though being a much smaller object, can almost block out the entire sun because the Moon is so much closer! https://www.timeanddate.com/eclipse/in/canada/red-deer Naked−Eye Observations · 2 − 16 · 2.3 The Moon ASTR 1205 Unit 2 Dr. Bryan Rowsell 2.4 Ancient Mystery of Planets Planets behave just like starts (to the naked eye they are hard to distinguish). However planets have a very complex motion compared to the distant stars of the celestial sphere, they are not fixed in the night sky, they move or “wander” (that’s the Greek origin of the word Planet). Planetary motion was very hard to figure out, as all of the planets have different sizes, brightness, distances and orbital planes off the ecliptic (see p. 1−9). Instead of moving steadily eastward across the sky, planets can appear to reverse course, called retrograde motion, for a few weeks or a month. Naked−Eye Observations · 2 − 17 · 2.4 The Ancient Myster of Planets ASTR 1205 Unit 2 Dr. Bryan Rowsell You can see this makes the idea of an Earth−centered Solar System pretty impossible. Another problem facing ancient astronomers was parallax error. The further the object, the smaller the parallax effect. If the Earth is stationary, there should be no parallax effect. None was observed until the modern era, so it’s not that the Earth is stationary, it’s that the objects in the sky are so far away that the parallax effect is very small! Naked−Eye Observations · 2 − 18 · 2.4 The Ancient Myster of Planets ASTR 1205 Unit 2 Dr. Bryan Rowsell Chapter 2: The Essential Cosmic Perspective Visual Skills Check: 1−8. Solutions are found on Bb. End−of−Chapter Questions: 2−16, 19−36. Solutions are found on Bb. Extra Resources: Watch the following Crash Course Astronomy videos: Naked Eye Observations [11 min]: https://www.youtube.com/watch?v=L-Wtlev6suc Cycles in the Sky [10 min]: https://www.youtube.com/watch?v=01QWC-rZcfE Moon Phases [10 min]: https://www.youtube.com/watch?v=AQ5vty8f9Xc Eclipses [11 min]: https://www.youtube.com/watch?v=PRgua7xceDA Download the free Stellarium app on your Android phone or install the free program on your computer: https://stellarium.org/ (iPhone users also have access to the app, but it's not free, you can try Night Sky as well) Learn how to estimate how much time until sunset using your new knowledge of angular distances: https://www.wikihow.com/Find-Out-How-Much-Time-Is-Left-Before-Sunset Snack & Study by doing this Oreo Cookie Moon Phase activity: https://sciencebob.com/oreo- cookie-moon-phases/ Read this article about the Longitude Problem if you love history, navigation, clocks, and/or sailing: https://longitudeprize.org/the-history Chuckle at this PG-13 xkcd comic: https://xkcd.com/1020/ Review your understanding of the reason for the seasons: https://spaceplace.nasa.gov/seasons/en/ Watch this video about retrograde motion [4 min]: https://www.youtube.com/watch?v=FtV0PV9MF88 Naked−Eye Observations · 2 − 19 · Questions and Resources