CL - P & C (remote)[17]-1 (1)

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Astronomy

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Apr 30, 2024

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Computer Lab – Planets & Conjunctions (Virtual Lab Remote Edition) Discovery of Neptune [Parts of this discussion were adapted from Scientific American , Dec. 2004 and DIO , June 1999] "That star is not on the map!" Those were the famous words of astronomy student Heinrich d'Arrest to staff astronomer Johann Galle at the Berlin Observatory announcing the discovery of Neptune. Galle was testing an extraordinary prediction made by French mathematician Urbain Jean Joseph Leverrier. Leverrier was investigating the motion of Uranus, the outermost known planet of the time. To account for the observed path of Uranus, he had hypothesized an eighth planet and calculated where such a planet must be to explain the path of Uranus. He had determined the necessary location for such a planet and sent that information to Galle. In less than half an hour of observing on September 23, 1846, Galle spotted a small blue disk very close to Leverrier's prediction. Leverrier later named the planet Neptune. Start the Voyager 4 program and click the ABC button at the right edge of the screen to turn off labels (turn the button from black to white). Select the Display/Magnitude Limits… menu and move the left slider vertically so that the limit for Narrow Field reads 11.0. Click the OK buttons to dismiss the window. Now set your observing location to Berlin in Europe (I believe you can do this without my giving you detailed instructions). Set the date to Sept. 23, 1846 at, say, 11 PM. Zoom to 1°, this gives a view on screen about six times as wide as the telescopic view that Galle had but we don't want your search to take half an hour. Select the Center/Coordinates… menu and select the Ecliptic button. Enter 325,0,0 for Longitude, 0,0,0 for Latitude. Click OK. This is where Leverrier said to look but Neptune does not appear in this view. So you need to move your 'telescope' around a bit. Hold down the spacebar on the keyboard and click and drag within the Voyager window (hold the mouse button down while moving the mouse). The cursor on screen turns into a hand and you can slide the field of view around. Search until you find Neptune, a blue spot larger than any stars. P & C – 1

John Couch Adams, a young British mathematician, was working on the Neptune calculation just like Leverrier. About a week after Galle had found Leverrier's planet, British Astronomer Royal George Airy announced the following; (a) Adams had also successfully calculated Neptune's position; (b) Adams was shy and had delayed communicating or publishing his results; (c) British astronomer James Challis had also now found Neptune using Adam's results; and (d) Challis had delayed his search because he lacked the detailed star charts available in Berlin. Although skeptics doubted many of these claims, Adams was given credit as the co-discoverer of Neptune along with Leverrier. The true story was not uncovered until October of 1998 when Olin Eggen died in Chile (he was the current assistant Royal Astronomer and had taken secret documents with him to an observatory in Chile). This "Neptune File" has shown that all of Airy's claims were untrue. Adams had not made any accurate calculations. Adams was in close contact with Airy and Challis. Challis had been actively searching for Neptune for months and did have the same star charts available in Berlin. Challis did locate Neptune six days after Galle and d'Arrest, but only when he searched around the location calculated by Leverrier! While it is true that one of Adam's many calculated positions for Neptune was very close, this cannot be considered a real prediction. In fact, Adam's best value prior to Neptune's discovery was a position off by a whopping 12° and Adam's had so little confidence in the result that he did not want to publish it. The British, primarily Airy, were loathe to cede the glory of Neptune's discovery to France and inflated the accuracy of Adam's calculations to rival those of Leverrier and threw out a variety of excuses why no successful search had been carried out in England. The British were even so brazen as to push their favored name for the planet of Oceanus instead of Neptune. Now that the truth is known, it is clear that only Leverrier deserves credit for the discovery of Neptune and that Airy and his successors that perpetuated the lie deserve condemnation. 1. Check the Info Panel for Neptune, what were the Ecliptic Longitude and Latitude values of Neptune when discovered? Ecliptic Lon.: 325 degrees 52' 35.7" Ecliptic Lat.: -00 degrees 31' 57.3" P & C – 2

A Trip to Mars Select the File/Open Settings… menu, navigate through the folders Local Disk (C:), Program Files, Carina Software, Voyager 4, 110 Settings, and open the "Inner Planets" file. Start the animation. How do we get a spacecraft to go from Earth to Mars when they're both moving? We could blast the rocket so fast that it gets to Mars before Mars can ‘get away’. But we don’t have rockets like that and it would waste tremendous amounts of fuel. There must be some more efficient route. We want to get a spacecraft from Earth to Mars, the straight-line path is no good because the planets move, and it would require too much fuel (even when they are relatively close to each other). The most fuel-efficient trajectory from one orbit to another (assuming no "gravitational slingshots" are available) is called a Hohmann transfer. We can create such an orbit on screen. (A gravitational slingshot is a way of getting a boost for a spacecraft by having fly past a planet, these are often used to get spacecraft out to Jupiter, Saturn and beyond, but rarely used in trips to Mars.) Stop the animation, click on the Now button on the Time Panel. Select the Tools/Planet Report… menu, "Heliocentric Positions" should be selected in the pop-up menu by default. Record the Longitude and Distance values for Earth. Longitude = 184.0729 Distance (AU) = 0.99700 The longitude will be a value between 0 and 360° (you can round it to the nearest degree) while the distance should be very close to 1. Close the Planet Report window. Select the Tools/Define Orbiting Object… menu. Type in any name you want for your spacecraft. Leave the number at 0, make the Type a spacecraft, leave the Primary as Sun, and don't change the Diameter or Mag. Params. In the "Orbit Size and Shape" area, we want the Perihelion Distance to be the Distance value you wrote down above. Use 0.21 for the Eccentricity; this combined with the perihelion distance will give an aphelion distance of about 1.53 AU, right around the orbit of Mars. Leave the Drag Coefficient at 0. In the "Orbit Orientation and Position in Orbit" area, leave the Equinox at 2000. Enter 0 for the Inclination (so your spacecraft will stay in the ecliptic plane), 0 for Longitude, and then for the Argument of Perihelion enter the Longitude value for Earth that you wrote above (this means the place where the spacecraft is closest to the Sun – perihelion – is exactly where Earth is now). Change the Mean Anomaly to zero. For the Year, Month, and Day we want the current year, month, and day; they are probably already correctly filled in. It should P & C – 3

look like the picture below but with the empty boxes filled in as above. Click OK. Select the Window/Planet Panel menu and click on the Spacecraft tab. You should see your spacecraft on the list, click on the Name, Sym(bol), and Orbit boxes to turn those on for your spacecraft. Close the Planet Panel. Your spacecraft should appear atop the Earth and you should see its green orbital path arcing away from the Earth out to the vicinity of Mars' orbit. If it isn't correct, select the Tools/ Select Satellites and Spacecraft… menu, click on your spacecraft, then click Edit…. You'll need to select Perihelion Distance for the left pop-up P & C – 4

menu, then double-check all the values for the spacecraft. Your spacecraft's orbit may have come up short or gone past Mars' orbit. That's because Mars has a rather elliptical orbit and is sometimes closer or further than its average distance. We could try to correct it by editing the spacecraft's Eccentricity value, or we can just ignore it. All right, we’ll ignore it. Animate. A spacecraft launched from Earth will already be moving in the same direction as Earth with the same speed. The spacecraft already has that velocity for 'free', an efficient trip to Mars will just add a little extra to the velocity that the spacecraft already possesses. In a Hohmann transfer, the rocket propelling the spacecraft accelerates it in the same direction that the Earth (and it) were already moving. With that extra speed, the spacecraft arcs into a wider orbit, reaching the orbit of Mars half an orbit later. Our goal was to get the spacecraft to Mars, not just to the orbit of Mars. Was Mars anywhere near the spacecraft when the spacecraft reached Mars' orbit? You can click Now to reset and animate again to check if you’re unsure. We need to launch the spacecraft from Earth at a time such that when it reaches aphelion, Mars will be right there. These Earth-to-Mars “launch windows” come roughly every two years. Click once on your spacecraft to get its info Panel. The spacecraft goes from Earth at perihelion to Mars (or at least Mars’ orbit) at aphelion, that’s half of a full orbit. So the time to get from Earth to Mars will be half the spacecraft’s full period. How long will the Earth-to-Mars trip take for your spacecraft in days ( again, this is half the Orbital Period listed for the spacecraft )? Also convert to months by dividing the days by 30. Travel Time = 258.925 days = 8.63083 months Open the Settings File "CSUB-Mars" in the 110 Settings folder. The left window shows "Roadrunner-1" on the day it is launched from Earth. Note where Earth and Mars are in their orbits, this is a correct launch window to get to Mars. Roadrunner-1 starts out moving at Earth's fast speed plus some extra speed. It slows as it arcs out towards Mars and is going slower than Mars when they meet. To go into orbit of Mars or land on Mars, the spacecraft would have to accelerate to match orbits when it reaches Mars. The two windows on the right show views from the spacecraft locked on Mars and Earth. Start the animation and watch the journey. Stop when the spacecraft is close to Mars. The two little white dots buzzing around Mars are its two small moons. 2. What are the names of the two moons of Mars? Phobos and Deimos Maximum Elongation P & C – 5

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