Thursday, September 29, 2022

The Ellensburg, WA sky for the week of October 1, 2022

Saturday: Since Halloween is later this month; the stores are filled with bags of candy clusters. Use this reminder to take time to look at a star cluster. The Hyades cluster is an open star cluster that represents the V-shaped face of Taurus the bull. It is one of the biggest and nearest star clusters with about 200 stars 150 light years away. The Hyades cluster was the first cluster to be the subject of detailed motion studies. These studies allowed astronomers to pinpoint the distance to the Hyades and provide important information about the scale of the universe. Aldebaran, nearly two and a half fists held upright and at arm’s length above the eastern horizon at midnight, is a foreground star and not a part of the Hyades cluster.

Sunday: According to the “One world, group hug, love everyone” philosophy, political borders are human-made and can’t be seen from space so why can’t we all just get along. According to real world pragmatic discoveries, some human-made political borders CAN be seen from space. Since 2003, India has illuminated its border with Pakistan to prevent illegal crossings. In 2011, astronaut Ron Garan took a picture of that border from the International Space Station. For more information, including the photo, go to http://goo.gl/mY8xG.

Monday: At 10:00 p.m., Uranus is two fists above due east and two fists below the bright, orange-ish star Hamal. You’ll know you have found the planet in your binoculars when you revisit that location for the next few nights and see that the point of light has moved up and to the right.

Tuesday: Coffee. First scientists say it’s good for you. Then they say it is bad for you. Recently, the same argument was applied to an exomoon, a moon orbiting a planet outside our Solar System. No, astronomers are not debating whether exomoons are good for you. Of course they are. But there are conflicting reports over whether the initial exomoon observation shared a year ago was real or just a blip in the data. Astronomers studied the light of a star as a Jupiter-sized planet and then its Neptune-sized moon blocked it. This transit method is one of the most popular ways to observe exoplanets… and maybe exomoons. Read more about the debate at https://www.sciencealert.com/the-first-known-exomoon-is-called-into-question-in-follow-up-studies.  

Wednesday: Saturn is less than a fist to the upper right of the Moon at 10:00 p.m. in the southern sky. At this same time, Jupiter is nearly three and a half fists above due southeast and mars is rising above the east-northeastern horizon.

Thursday: In 1987, the rock group Def Leppard sang “Pour some sugar on me, in the name of love. Pour some sugar on me, come on, fire me up”. In 2012, some European astronomers “found some sugar near stars, they were very young. Found some sugar near stars, out where planets formed.” Astronomers observed molecules of glycolaldehyde, a simple form of sugar, in the disk of gas and dust orbiting young binary stars. This is the first time astronomers have found this simple sugar so close to a star indicating that organic molecules can be found in planet-forming regions of stars. For more information, go to http://goo.gl/tfwy1.

Friday: Mercury is a fist above the eastern horizon at 6:30 a.m.

The positional information in this column about stars and planets is typically accurate for the entire week. For up to date information about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm.

Thursday, September 22, 2022

The Ellensburg, WA sky for the week of September 24, 2022

Saturday: Saturn is two and a half fists held upright and at arm’s length above due south at 9:30 p.m. At this time, Mars is just rising above the east-northeastern horizon.

Sunday: Ask someone which day in September has the same duration day and night. Go ahead, ask someone. Why are you still reading this? I can wait. If that person says the first day of autumn, they are wrong. Today, three days after the first day of autumn, is the date in which day and night are closest in duration. There are two main reasons for this. First, the atmosphere acts like a lens, bending light from the Sun above the horizon when the Sun is actually below the horizon. This makes the Sun appear to rise before it actually rises and appear to set after it actually sets. Second, fall starts when the center of the Sun passes through the point called the autumnal equinox. But, the Sun is not a point. The upper edge of the Sun rises about a minute before the center of the Sun and the lower edge sets a minute after the center of the Sun. Thus, even if we didn’t have an atmosphere that bends the sunlight, daytime on the first day of autumn would still be longer than 12 hours.

Monday: Jupiter is in opposition tonight. That doesn’t mean that Jupiter is a teenager. Opposition means that Jupiter is on the opposite side of the Earth as the Sun. When an object is in opposition, it is at its highest point in the sky during the darkest time of the day. Thus, opposition is typically the best time to observe a planet. Jupiter is four fists above due south at 1 a.m., midnight non-daylight savings time. If you don’t want to stay up so late, you can see it three fists above due southeast horizon at 9:30 p.m.

Tuesday: To the people of Ancient Greece, the stars that are about five and a half fists above the northeastern and eastern horizons, respectively, at 10:00 p.m. were known as Cassiopeia and Andromeda, a mythological queen and her daughter. But not all cultures imagined the same pictures in the sky. To the people of Polynesia, the stars of Cassiopeia and Andromeda represented a dolphin, called Kwu. Cassiopeia formed its tail, the brightest stars of Andromeda formed its fins, and its fainter stars outlined the dolphin’s body. 

Wednesday: You discovered Cassiopeia last night. The astronomer Caroline Herschel discovered an open star cluster that looks like a rose over 200 years ago. This cluster, called Caroline’s Rose, is about 6,500 light years away and consists of about 1,000 stars that are one third the age of the Sun. Through binoculars, it looks like a fuzzy patch. At 10:00 p.m., find the star at the top of the sideways “W” known as Cassiopeia. When that star is in the lower left portion of your field of view, Caroline’s Rose is in the center to upper right. For more information about Caroline’s Rose, go to http://tiny.cc/i0zxsz and have the story read to you.

Thursday: Mnemonics are helpful for remembering astronomy facts. (Similarly, “Johnny Mnemonic”, the 1995 cyberpunk film, was helpful in getting Keanu Reeves’ career going.) After all, school children around the world are learning the order of the planets by remembering, “My very excellent mother just served us nine….” Oops, I guess that one needs updating. Well, here’s one that will not need updating for tens of thousands of years: the order of the stars in the Big Dipper. Because the nighttime stars are so far away from us, their actual motion through the sky, called their “proper motion” is not noticeable over even thousands of years. That is why the constellations have remained the same since ancient times. But two stars in the Big Dipper have a proper motion large enough such that in 100,000 years; the stars will no longer make a dipper shape. You can see this simulation at the American Museum of Natural History video found at https://youtu.be/sBfUBtdo8yo. Until then, you can remember the names of the seven dipper stars in order from handle to cup by remembering this helpful advice for teens: “AM, ask mom. PM, dad”. The stars are Alkaid, Mizar, Alioth, Megrez, Phad, Merek, and Duhbe. The Big Dipper is one fist held upright and at arm’s length above the northern horizon at 11:00 p.m. 

Friday: “There’s water in them thar craters”, frozen water, that is. There has been speculation since the 1960s and indirect evidence since the 2000s of water on the Moon. Three years ago, astronomers studied data from four earlier missions. They noticed that the  light reflecting off the bottom of craters near the lunar South Pole showed characteristics of light reflecting off pure ice. The water likely came from comet impacts or other solar system objects with trace amounts of water ice. Last week, NASA announced the landing site for its new Volatiles Investigating Polar Explorer Rover or VIPER, the mission that will take a close-up view of Nobile crater near the Moon’s South Pole. For more information about the upcoming mission, watch the video at https://youtu.be/bd7ekqMrHkg.

The positional information in this column about stars and planets is typically accurate for the entire week. For up to date information about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm.

Thursday, September 15, 2022

The Ellensburg, WA sky for the week of September 17, 2022

Saturday: Jupiter is one and a half fists held upright and at arm’s length above the east-southeastern horizon and Saturn is about two fists above the south-southeastern horizon at 9:00 p.m. Jupiter is the much brighter of the two. When you are looking at this part of the sky, you are looking in the direction of more than just the two planets. You are also looking in the direction of their moons. Titan, Saturn’s largest moon, is easily visible with a small telescope, about six “ring diameters” to the side of Saturn at this time. Jupiter’s four largest moons are also visible with a small telescope. Callisto, Ganymede, and Io are one one side of Jupiter, with Callisto appearing the farthest away, Ganymede in the middle, and Io closest. Europa is visible in the other. Last year, a team of Canadian astronomers analyzed images of Jupiter from 2010 and estimated that Jupiter could have 600 moons at least 800 meters, a half mile, in diameter. They didn’t actually discover these moons. They just formulated a possible model of the Jovian system. For more on this, go to https://skyandtelescope.org/astronomy-news/jupiter-could-have-600-moons/.

Sunday: Two years ago, astronomers announced that they detected phosphine, a possible biosignature of life, in the upper atmosphere of Venus. Although the surface of Venus is inhospitable, astronomers have long speculated that the upper atmosphere could harbor life. Not Cloud City life from The Empire Strikes Back but maybe microbial life. Last year, astronomers concluded that the original scientists found the signature of sulfur dioxide, not life. Last month, the private company Rocket Lab published details about the first privately funded mission to another planet - their trip to Venus, scheduled to launch as soon as May 2023. This Venus storyline is an excellent example of science at work. In less than two years, science went from “we may have found a marker for life on Venus” to “it is unclear if we found a marker for life” to “we probably didn’t find a marker for life” to “let’s visit Venus to closely study the thing we thought was a marker for life”. Do an internet search of the words venus and phosphine and read the articles to follow the story. To get yourself in the mood, go outside at 6:15 a.m. Venus is just above the eastern horizon at this time.

Monday: The bright star Vega is about five fists above the western horizon at 11:00 p.m. Its fellow Summer Triangle star Deneb is about two fists above it. Altair, the third star in the triangle, is about four fists above due southwest.

Tuesday: The bright star Capella is two fists above due northeast at 11:00 p.m.

Wednesday: The Beehive Cluster is less than a half a fist to the lower right of the waning crescent Moon at 5:00 a.m. They are about three fists above due east.

Thursday: At 6:06 p.m. PDT, the center of the Sun crosses the celestial equator and passes into the southern sky. The celestial equator is an imaginary line that divides the sky into a northern and southern half. When the Sun is in the southern half of the sky, it appears to take a shorter path from rising to setting. It also does not get as high in the sky at noon. This leads to shorter days and longer nights. Since the Sun crosses the celestial equator today, there is an instant when it is equally in the northern and southern sky, called the north and south celestial hemispheres. This so-called “equal night” is given by the Latin word equinox. Thus, today is known as the Autumnal Equinox. However, the day and night are not of equal duration today. The sun rises at 6:49 a.m. and sets at 6:59 p.m. in the northern latitudes of the United States. At these latitudes, day and night are closest to equal duration on Sunday.

Friday: The bright star Sirius is two fists above the south-southeastern horizon at 6:00 a.m.

The positional information in this column about stars and planets is typically accurate for the entire week. For up to date information about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm.

Thursday, September 8, 2022

The Ellensburg, WA sky for the week of September 10, 2022

Saturday: Watch the Moon move closer to Jupiter throughout the night. When they rise at about 8:00 p.m., Jupiter is about one fist to the left of the Moon. At midnight, Jupiter is a little more than a half a fist to the upper left of the Moon in the southeastern sky. By 6:00 a.m. tomorrow morning, Jupiter is less than a half a fist above the Moon, low in the west-southwestern sky. If you’d like the challenge of seeing a planet in the daylight, wait another hour until 7:00 a.m. First find the Moon very low in the western sky. Use binoculars to look at the Moon, making sure the Moon is in the lower part of your field of view. Jupiter will be above the Moon, in about the middle of your field of view. Then move your binoculars away and look right above the Moon with your naked eyes. You may be able to see Jupiter.

Sunday: “You know Aries and Cancer and Draco and Libra. Leo and Pisces and Virgo and Hydra. But, do you recall, the pointiest asterism of all? Triangulum, the three sided asterism, had a very pointy shape. And if you didn’t know it, you would say it poked an ape.” Sorry. Some stores have started sending out their Christmas catalogs and that has put me in the mood to modify some Christmas songs. Anyway, Triangulum is a small constellation between the more prominent Andromeda and Aries. Its main feature is a skinny triangle oriented parallel to and nearly four fists held upright and at arm’s length above the eastern horizon at 11:00 p.m.

Monday: Fomalhaut, the southernmost bright star visible from the northern USA, is one fist above the south-southeastern horizon at 11:00 p.m. In 2008, Fomalhaut and its surroundings became the first star system with an extrasolar planet to be directly imaged. See the family photo at  https://apod.nasa.gov/apod/ap081114.html

Tuesday: While many people think they need a telescope to enjoy looking at the night sky, some objects actually look better through binoculars. Open star clusters are one of those types of objects. M39, the 39th object in Charles Messier’s catalog, is straight overhead at 11:00 p.m. This open star cluster contains about 30 stars in a region about seven light years across and a thousand light years away. A quick trigonometry calculation shows that the cluster is about the same size as the full moon in the night sky. Read more about M39 at https://stardate.org/radio/program/2022-09-04.

Wednesday: At 11:00 p.m., Saturn is two and a half fists above due south, Jupiter is three fists above southeast, and Mars is less than a half a fist above the east-northeastern horizon.

Thursday: Earlier this week, you read about Fomalhaut, the second brightest star with a planet. The brightest star known to have a planet is Pollux, in the constellation Gemini. (First vs. second brightest is meaningless here because they are nearly identical in magnitude, 1.15 vs. 1.16.) Pollux is four and a half fists above due east at 5:30 a.m., right below its “twin” star Castor.  Read more about Pollux at https://goo.gl/cL5t9p.

Friday: Neptune is in opposition tonight. Opposition doesn’t mean that Neptune disagrees with everything. It means that it is on the opposite side of the Earth as the Sun. When an object is in opposition, it is at its highest point in the sky during the darkest time of the day. Thus, opposition is typically the best time to observe a planet. Neptune is a little more than three fists above the southeast horizon at 11:00 p.m. It is one fist to the right of bright Jupiter.

The positional information in this column about stars and planets is typically accurate for the entire week. For up to date information about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm.

Thursday, August 25, 2022

The Ellensburg, WA sky for the week of September 3, 2022

Saturday: Jupiter and Saturn line up in the southeastern sky at 11:00 p.m. Saturn is two and a half fists held upright and at arm’s length above the southern horizon and Jupiter is two and a half fists above the eastern horizon. 

Sunday: “Excuse me, do you have the time?”

“No, but the Big Dipper does.”

You can use the orientation of the Big Dipper to tell time with a precision of about 15-30 minutes. First, find the two stars at the far end of the Big Dipper cup, the stars that do not touch the handle. Draw an imaginary line segment starting at the North Star and passing through the two Big Dipper cup stars. Now, draw a big circle around the North Star. Your circle is a 24-hour clock. Number the circle from 0 hours at the top, counterclockwise to 12 hours at the bottom of the circle, and back up to 24 hours at the top. (O hours and 24 hours are the same on this clock because the day is 24 hours long.) The hour number on the big circle closest to where your imaginary line intersects this circle is called your raw time. Due to the location of the Big Dipper compared to the rest of the stars, the time nearest the intersection (the raw time) is correct for March 6. For any other night, subtract two times the number of months the current date is after March 6 from the raw time. For example, let’s say the imaginary line between the North Star and the Dipper stars is pointed to the right. That means the raw time is 18 hours or 6 p.m. If you made this observation tonight, which is six months after March 6, you would subtract two times six or 12 hours from the raw time.  Don’t forget to convert for daylight savings time if needed. For a more complete explanation on how to do the Big Dipper clock math, go to http://goo.gl/02HmA. If you prefer a more visual tool, and a fun project to do with your kids, there is a simple “star clock” template and instructions at https://skyandtelescope.org/observing/make-a-star-clock/. Use this paper star clock whenever your watch is broken. The Big Dipper is in the northwestern sky at 9:00 p.m. tonight.

Monday: Labor Day was the brainchild of labor unions and is dedicated to American workers. The first Labor Day was celebrated in 1882. The Greek mythical hero Hercules probably wished there was a Labor Day to commemorate his work. As punishment for killing his family while he was temporarily insane, he had to perform twelve nearly impossible tasks such as killing monsters or stealing things from deities. Hmmm. Maybe we shouldn’t commemorate his labors. But we can enjoy his constellation. The keystone asterism representing the body of Hercules is six fists held upright and at arm’s length above the western horizon at 10:00 p.m. For more information about the Labors of Hercules, go to http://goo.gl/ozVF5.

Tuesday: The asteroid Juno is in opposition tonight. That doesn’t mean that Juno says “no” when you say “yes”. Opposition means that Juno is on the opposite side of the Earth as the Sun. When an object is in opposition, it is at its highest point in the sky during the darkest time of the day. Thus, opposition is typically the best time to observe an asteroid or planet. Pallas is three fists above the southeastern horizon at 11:00 p.m. First find Jupiter and Saturn two and a half fists above the southeastern and southern horizon.

Wednesday: School starts this week so it is time for a little geometry review. A square is a regular quadrilateral. This means it has four equal sides, four equal angles, and wears old fashioned clothing. Go outside at 10:00 p.m. tonight with a notebook in hand to sketch one. The Great Square of Pegasus is balancing on its corner three fists above the eastern horizon. The top corner of the square is two fists above the bottom corner. The other two corners are to the left and right of the line segment connecting the top and bottom corners. 

Thursday: Geometry review, part 2. Go outside at 10:00 p.m. tonight with a notebook in hand. (Good teaching involves a little repetition.) A triangle is a polygon with three corners and three line segments as sides. A good example is the Summer Triangle made up of the bright stars Vega, Deneb and Altair. Altair is five fists above the southern horizon. Vega, the brightest star in the triangle, is seven fists above the western horizon. Deneb is a little bit east of straight overhead.  

Friday: Mars is about a half a fist above the east-northeastern horizon at 11:30 p.m.

The positional information in this column about stars and planets is typically accurate for the entire week. For up to date information about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm.

The Ellensburg, WA sky for the week of August 27, 2022

Saturday: Jupiter and Saturn line up in the southeastern sky at 10:00 p.m. Saturn is two fists held upright and at arm’s length above the southeastern horizon and Jupiter is one fist above the eastern horizon. Astronomers discovered volcanoes spewing water vapor on Saturn’s moon Enceledus back in 2005. Just two years ago, scientists published findings of similar activity on Jupiter’s moon Europa. It’s not possible to see Enceladus with a small telescope. But you can see Europa using a small telescope.  If you look at 10:00 p.m., you’ll see all four large moons: three on one side and Europa on the other, about four Jupiter diameters from Jupiter. For more information about Europa’s geysers, go to https://earthsky.org/space/europa-water-vapor-geysers-goddard

Sunday: Mars is about a half a fist above the east-northeastern horizon at midnight.

Monday: Had the script been written a little differently for a well-known Robin Williams movie, we might have heard Mr. Williams shout, “Goooood Morning Orion the hunter”. Orion is typically thought of as a winter constellation. But, it makes its first appearance in the early morning summer sky. The lowest corner of Orion’s body, represented by the star Saiph (pronounced “safe”), rises at 2:00 a.m. By 5:30 a.m., Orion’s belt is three fists above the south-southeastern horizon. 

Tuesday: Need a caffeine pick-me-up? Make it a double. Need an astronomy pick-me-up? Make it a double double. Find Vega, in the constellation Lyra the lyre, nearly straight overhead at 9:00 tonight. Less than half a fist to the east (or left if you are facing south) of the bright bluish star Vega is the “star” Epsilon Lyra. If you look at Epsilon Lyra through binoculars, it looks like two stars. If you look at Epsilon Lyra through a large enough telescope, you will notice that each star in the pair is itself a pair of stars.  Each star in the double is double. Hence, Epsilon Lyra is known as the double double. The stars in each pair orbit a point approximately in the center of each respective pair. The pairs themselves orbit a point between the two pairs.

Wednesday: While you were looking through your underwear drawer for clean socks, some citizen scientists were looking through sky maps obtained by robotic telescopes to find brown dwarfs. The project, called Backyard Worlds: Planet 9, allows anyone with a computer and internet connection to search through thousands of images to find these strange objects that are midway between being classified as large planets and small stars. Some brown dwarfs can have surface temperatures of thousands of degrees Celsius, the recently discovered ones are cooler than the boiling point of water and may even have clouds of water vapor! Read more about the discovery and how you can participate in this project at https://www.jpl.nasa.gov/news/news.php?release=2020-159

Thursday: “I’m a little teapot, short and stout. The galactic center, I pour it out.” (I’m a Little Teapot, astronomy version, 2021.) Despite its great size and importance, the center of our Milky Way galaxy and its giant black hole remains hidden to the naked eye behind thick clouds of gas and dust. By plotting the orbits of stars near the middle of the galaxy, astronomers have determined that the black hole’s mass is equal to about 4.5 million Suns. While you can’t see the actual galactic center, you can gaze in the direction of the center by looking just to the right of the teapot asterism in the constellation Sagittarius. This point is about one fist above the south-southwestern horizon at 9:00 p.m.

Friday: One of the most important lessons to learn in science class is to always save your data. Even if the data looks “wrong” or you don’t think it contains important information, you never know what future researchers might find in it. Two years ago, Kepler space telescope scientists reanalyzed older data and discovered an Earth-sized planet orbiting in the habitable zone of its host star. It turns out the previous computer algorithm misidentified it. Maybe there are other Earth-cousins hiding in the data somewhere. Read more about it at https://www.nasa.gov/press-release/earth-size-habitable-zone-planet-found-hidden-in-early-nasa-kepler-data.

The positional information in this column about stars and planets is typically accurate for the entire week. For up to date information about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm

Wednesday, August 17, 2022

The Ellensburg, WA sky for the week of August 20, 2022

Saturday: “The sky is black (or light polluted), the stars are white (or red or orange or yellow or blue), the whole world gazes upon the sight (except where there are too many city lights or people are lazy.” Wow. It is difficult to write a flowing set of lyrics when there are so many parenthetical thoughts. Most people think of the sky’s blackness as a lack of stars. But dark patches in the Milky Way are actually massive clouds of dust that are blocking the stars behind them. Two of the most prominent are dark nebulae B142 and B143 in the constellation Aquila the eagle. These are easy to find and enjoy with binoculars. First find the bright white star Altair, five fists held upright and at arm’s length above the southeastern horizon at 10 p.m. Then move your binoculars up a little bit to the next bright star Tarazed, about one fifth as bright. B142 and B143 are to the upper right of Tarazed. They make an “E” shape in the sky; fitting because American astronomer E. E. Barnard first proposed that these were dust clouds and not simply big spaces between the stars. For more information about dark nebulae, including many more to look at with binoculars, go to https://goo.gl/9tiqdh.  

Sunday: Arcturus is two and a half fists held upright and at arm’s length above the western horizon at 10:00 p.m. This star, whose name means bear watcher, is the brightest in the sky’s northern hemisphere. It follows Ursa Major, the Great Bear, around the North Star. Arcturus is the closest giant star to Earth. It is one of the few stars whose diameter can be measured directly rather than being inferred from its density and mass, which themselves are derived from other parameters. 

Monday: Saturn is two fists above the southeastern horizon at 10:30 p.m. Jupiter is one fist above the eastern horizon at this time.

Tuesday: Deneb is straight overhead at 11:30 p.m. When you look at Deneb, you are seeing light that left Deneb about 2,600 years ago.

Wednesday: All stars rotate. Our Sun takes a little less than one Earth month to rotate once on its axis. Astronomers studied the relationship between mass, stellar rotation, and planetary formation by aiming NASA’s recently retired Kepler space telescope toward the Pleiades open star cluster. All 1,000 stars in this group are nearly the same age, 125 million years old. Since all of the stars are the same age and formed from the same set of materials, astronomers have the ideal “laboratory” to isolate the role star mass plays on star rotation and evolution. Read more about the findings at http://goo.gl/osijIY. See the Pleiades for yourself, a half a fist above the east-northeastern horizon at 11:15 p.m.

Thursday: Seventeenth century astronomers documented the appearance of a new star, or “nova”, in 1670. However, as modern astronomers studied the records of the star, called Nova Vulpeculae 1670, they realized it didn’t have the characteristics of a typical nova because it didn’t repeatedly brighten and dim. It brightened twice and disappeared for good. Turning their telescopes to the region, they discovered the chemical signature to be characteristic of a very rare collision of two stars. For more information about this discovery, go to http://goo.gl/rJnC2G. Nova Vulpeculae 1670 is right below the binary star system Alberio, the head of Cygnus the swan. Alberio is seven fists above due south at 10:15 p.m.

Friday: Mars is finally rising at a semi-reasonable hour. It is a half a fist above the east-northeastern horizon at midnight.

The positional information in this column about stars and planets is typically accurate for the entire week. For up to date information about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm.