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AstroGrav Blog

The following articles form a blog about astronomical subjects that are ideal for investigation and study with AstroGrav. They are ordered chronologically, with the most recent first and the oldest last. Most of the articles include illustrative movies or screenshots, although the image quality of the movies is not as good as when running simulations from within AstroGrav.

2017

2016

If there's any topic that you'd like to see covered in a future blog article, then please contact us and we'll look into it.

The Exoplanets Orbiting Kepler-90

Published on 15th December 2017.

This is an AstroGrav video that shows a simulation of the Kepler-90 planetary system, as described in many media articles in December 2017.

This system was in the news on 14th December 2017 because it's the first exoplanet system found to have eight planets, like our solar system. It was already known to have seven planets, but an eighth has been discovered. The inner three planets are rocky (like the inner four of the solar system), the next three are ice giants (like Uranus and Neptune), and the outer two are gas giants (like Jupiter and Saturn).

The main difference from the solar system is that the distances are all on a much smaller scale, with the outermost planet about the same distance from Kepler-90 as we are from the Sun. As a consequence, most of the planets are very hot, and none of them are cold. All eight planets have almost circular orbits, and they're about 2,500 light years away, so quite distant. There's lots more information on Wikipedia.

The video is recorded at 24 frames per second with a time step of 1 day between each frame, so that the video runs at a speed of 24 days per second, and the whole video covers a period of about 2 years.

The Exoplanets Orbiting TRAPPIST-1

Published on 20th March 2017.

This is an AstroGrav video that shows a simulation of the TRAPPIST-1 planetary system, as described in many media articles in February 2017, and detailed in the Open Exoplanet Catalogue.

All seven planets have a similar mass and radius to the Earth, and have almost circular orbits.

The video is recorded at 24 frames per second with a time step of 2 hours between each frame, so that the video runs at a speed of about 2 days per second, and the whole video covers a period of about 60 days.

The Exoplanets Orbiting HR 8799

Published on 19th March 2017.

This is an AstroGrav video that shows a simulation of the HR 8799 planetary system, as described in many media articles in January 2017, and detailed in the Open Exoplanet Catalogue.

In some ways, this system is rather like a scaled-up version of Jupiter and its Galilean moons. The three inner planets (HR 8799 e, HR 8799 d, and HR 8799 c) have orbital periods in a 1:2:4 ratio, just like the three inner Galilean moons (Io, Europa, and Ganymede). The fourth planet (HR 8799 b) doesn't fit this pattern of period ratios, just like the fourth Galilean moon (Callisto).

The video is recorded at 24 frames per second with a time step of 2 years between each frame, so that the video runs at a speed of about 50 days per second, and the whole video covers a period of nearly 1,500 years.

Comets 73P and 73P-BT in March / April 2017

Published on 18th March 2017.

This is an AstroGrav video that shows a simulation of comets 73P and 73P-BT as seen from the Earth in March and April 2017. During this period, the two comets are near perihelion, which is just inside the Earth's orbit (0.97AU). They are about 90 degrees 'ahead' of the Earth in their orbit, with their distance from the Earth increasing from 1.4AU to 1.7AU during the period of the video. At closest approach on 27th March, the comets are slightly under two arcseconds apart.

To give an idea of scale and how close together these comets are, the Moon is about 60 times as wide as the view that is shown in this video. The blue lines show parts of the comets' orbits about the Sun.

The Distribution of Comets in the Solar System

Published on 31st May 2016.

This is an AstroGrav video that shows a simulation of all known comets in the solar system seen from various different viewpoints. The planets and their orbits are also shown to give an idea of relative scale.

Several concentrations of comets are evident. The concentration in the inner solar system is the result of comets being much easier to discover in the inner solar system, partly because comets are much brighter when nearer the Sun, and partly because such comets are also closer to the Earth. The other concentrations of comets are different groups of sungrazing comets, with the largest group consisting of over 1,200 Kreutz sungrazers. Smaller groups are Mayer sungrazers, Kracht sungrazers, and Marsden sungrazers.

Below are the instructions for setting up this simulation in AstroGrav.

  1. Open the 'Planets, Moon' sample simulation in the 'Solar System' folder.
  2. On the 'View' window (lower-left), select all the objects using the 'Edit / Select All' command.
  3. On the 'View' window, select 'View / Show Selected / Names' and 'View / Show Selected / Orbits'.
  4. On the 'View' window, deselect 'View / Show Selected / Data'.
  5. Select the 'Edit / Import Objects...' command.
  6. Select 'JPL Comets' and 'OK' from the 'Import Objects' dialog.
  7. Select all the comets in the table using 'Ctrl+A' on Windows or 'Command+A' on a Mac.
  8. Press the 'OK' button to start importing the comets. This will take several minutes to complete.
  9. On the 'View' window, deselect 'View / Show All / Names' and 'View / Show All / Orbits'.
  10. Use the 'File / Save As...' command to save the simulation to 'All Comets' or something similar.
  11. On the 'View' window, rotate and zoom as necessary.

The Trajectory of the Strange Object 2016 HO3

Published on 23rd May 2016.

This is an AstroGrav video that shows a simulation of the recently discovered strange object 2016 HO3 as viewed from the Earth, and as described in here.

The article is slightly misleading, since it says that the orbit of 2016 HO3 is locked into a figure-of-eight pattern. The orbit of 2016 HO3 is actually a one-year orbit similar to the Earth's, so that 2016 HO3 is always quite close to the Earth, and the figure-of-eight pattern is the path of 2016 HO3 against the background stars as viewed from the Earth. In the video, each time round the figure-of-eight takes one year, and the Sun can be seen flying past each time 2016 HO3 is near the bottom of the figure-of-eight. The brighter planets can also be seen in the video, but the Moon has been blacked out so that it doesn't appear flashing across the video.

The video is recorded at 24 frames per second with a time step of 5 days between each frame, so that the video runs at a speed of about 4 months per second, and the whole video covers a period of 10 years. The view shown is over 100 degrees square, which makes the distortion caused by mapping the celestial sphere onto a plane very obvious towards the edges.

The Exoplanets Orbiting Kepler-223

Published on 16th May 2016.

This is an AstroGrav video that shows a simulation of the Kepler-223 planetary system, as described in many media articles in May 2016, and detailed in the Open Exoplanet Catalogue.

This is a particularly interesting planetary system, as the planets appear to be locked in resonance. Each time the outermost planet (Kepler-223 e) orbits three times, the next planet inwards (Kepler-223 d) orbits four times, the next planet inwards (Kepler-223 c) orbits six times, and the innermost planet (Kepler-223 b) orbits eight times. All four planets are much larger than the Earth, yet are considerably closer to Kepler-223 than the planet Mercury is to the Sun.

The video is recorded at 24 frames per second with a time step of 3 hours between each frame, so that the video runs at a speed of about 3 days per second, and the whole video covers a period of about 89 days.

Mars and its Moons: 2015 to 2021

Published on 28th April 2016.

With the approaching opposition of Mars, this short AstroGrav video shows a simulation of Mars and its two moons, Phobos and Deimos, from 2015 to 2021.

You can watch the changing size, brightness, and phase of Mars, as well as the changing aspect of the moons' orbits. In particular, Mars appears much bigger and brighter than usual at the three oppositions in May 2016, July 2018, and October 2020. The Sun can be seen flashing past when Mars is at conjunction - on the opposite side of the Sun to the Earth - in July 2017 and September 2019. To give an idea of scale, the apparent diameter of the full moon would be about twelve times the width of this video.

New Insights Into the 1977 Wow Signal

Published on 22nd April 2016.

AstroGrav screenshot showing the 1977 Wow Signal

With regard to the recently proposed and much-publicised theory about the cometary origin of the 1977 Wow Signal, there's a very good reason why neither of the comets 266P/Christensen or 335P/Gibbs could be responsible - namely that neither of them was within 10 degrees of the location of the Wow Signal on 15th August 1977. This can very easily be verified by checking the positions of the comets using the "gold standard" - NASA JPL's HORIZONS system (see Note A below). The location of the Wow Signal was at RA 19h 27m (±2m), Dec -26° 57'. The location of 266P was at RA 18h 32m, Dec -7° 22'. The location of 335P was at RA 18h 39m, Dec -9° 38'. Neither of the comets is anywhere near the location of the Wow Signal.

Many popular astronomical software packages calculate the positions of comets very quickly by ignoring the gravitational influence of everything but the Sun. This can result in errors of many degrees over a period of 40 years, and incorrectly places comets 266P and 335P close to the location of the Wow Signal on 15th August 1977. In contrast, AstroGrav is a software package that is ideal for accurately calculating the positions of comets over long periods of time, since it correctly takes into account the gravitational influence of all the planets and major asteroids as well as the Sun.

Using AstroGrav, it is possible to simultaneously calculate the positions of thousands of comets in the NASA JPL catalogue on 15th August 1977 and show those near the location of the Wow Signal (see Note B below). Comets 266P and 335P are visible in the lower right of the attached screenshot, and there are no comets closer to the location of the Wow Signal than about three degrees. The positions of the comets can again easily be checked against NASA's HORIZONS system, and they're accurate to within a few arcseconds.

The closest comets to the location of the Wow Signal are C/2002 A1 and C/2002 A2. These are a particularly interesting pair of comets, as they've been calculated to have split from a single comet at around the time that the Wow Signal occurred. See http://iopscience.iop.org/article/10.1086/376977/pdf for details. Now we have a much more likely cometary candidate to explain the Wow Signal - comets 2002/A1 and 2002/A2. Are they the real culprits, or is it just a coincidence that they split from a single comet at about the right time and about the right place? At three degrees from the location of the Wow Signal and at a distance of over 20 astronomical units, it's very questionable. More research is needed, but one thing is certain - neither of the comets 266P or 335P was responsible for the 1977 Wow Signal.

Note A: NASA HORIZONS Instructions

  1. Go to the HORIZONS website at https://ssd.jpl.nasa.gov/?horizons.
  2. Click on the 'web-interface' link.
  3. Change the 'Target Body' to 'Comet 266P/Christensen' (or whichever comet you're interested in).
  4. Change the 'Location' to 'Geocentric'. [It makes little difference whether the location is at the centre of the Earth or at the surface.]
  5. Change the 'Time Span' to a 'Start Time' of '1977-08-15', an 'End Time' of '1977-08-16', and a 'Step Size' of one hour.
  6. Click on the 'Generate Ephemeris' button.
  7. Read off the right ascension and declination coordinates from the table.

Note B: AstroGrav Comet Import Instructions

  1. Download and install the latest version of AstroGrav from http://www.astrograv.co.uk/download.html.
  2. Open the 'Planets, Moon, Asteroids' sample simulation in the 'Solar System' folder.
  3. In the 'Units' window, change the 'Date' unit from 'local time' to 'universal time'.
  4. Use the 'Evolve Settings...' command in the 'Evolve' menu to change the mass threshold from 1.0e21 to 1.0e19 kilograms - this is to ensure that the gravitational influence of the asteroids are taken into account.
  5. Use the 'Evolve To...' command in the 'Evolve' menu to evolve the solar system to 'AD 1977 Aug 15, 12:00'.
  6. Select the 'Import Objects...' command the 'Edit' menu, select 'JPL Comets', and OK the dialog. You will then see a table of 3,400+ comets.
  7. Click on the 'Epoch' column header to sort the comets, scroll down to 'C/1801 N1 (Pons)' and select this and all the following 3,200+ comets, and OK the dialog. Ensure that the 'Fast Import' checkbox is NOT selected. [The earlier comets are omitted partly to speed up the import process and partly because the orbital elements of many of the earlier comets are only approximate.]
  8. AstroGrav will now spend the next several minutes importing the comets.
  9. On the right-hand view window, select the 'Invisibles' and deselect the 'Orbits' in 'View/Show All' submenu.
  10. Use the 'View From...' command in the 'View' menu to change the viewpoint to 'Center of the 'Earth'.
  11. Use the 'Find...' command in the 'Edit' menu to find the constellation of Sagittarius.
  12. Twice choose the 'Much Larger' command from the 'View/Magnification' submenu, and you will have the relevant region displayed at a suitable magnification.
  13. Use the various tools in the tool palette to show or hide star names, coordinate frames, etc.

Note: To import the comets ignoring the gravitational effect of everything but the Sun, repeat the above process with the 'Fast Import' checkbox selected in step (7). Step (8) will then be almost instantaneous, but very inaccurate.

The Changing Orbits of the Outer Planets

Published on 19th April 2016.

This video shows shows a simulation of the changing orbits of the outer planets from the present to slightly over a million years in the future. The perihelia and aphelia of the orbits are indicated with 'pi' characters and the ascending and descending nodes of the orbits are indicated with 'omega' characters. The other orbital elements are also changing with time, but this is not obvious in the video. Each frame of the video represents the passage of 1,000 years, so that each second of the video represents the passage of about 25,000 years.

The Changing Orbits of the Inner Planets

Published on 18th April 2016.

This video shows a simulation of the changing orbits of the inner planets from the present to slightly over a million years in the future. The perihelia and aphelia of the orbits are indicated with 'pi' characters and the ascending and descending nodes of the orbits are indicated with 'omega' characters. The other orbital elements are also changing with time, but this is not obvious in the video. Each frame of the video represents the passage of 1,000 years, so that each second of the video represents the passage of about 25,000 years.

A Planet in a Double Star System

Published on 3rd April 2016.

This is a fascinating video that demonstrates the chaotic motion of a planet in a double star system.

If you want to create something similar yourself with AstroGrav, create a new simulation and then use the following instructions.

  1. Edit the star to have the same radius, luminosity, and mass as the Sun.
  2. Create the second star with a mass half that of the Sun in a circular orbit with a period of 1.0 year.
  3. Create the planet in a circular orbit about the second star with a period of 0.2 years.
  4. Set the time step to (something like) one day.
  5. Run the simulation.

Comets 252P/LINEAR and P/2016 BA14 (PanSTARRS)

Published on 17th March 2016.

Comets 252P/LINEAR and P/2016 BA14 (PanSTARRS) have been in the astronomy news recently because of their current closeness to the Earth. Because of the similarity of their orbits, there has been some speculation that P/2016 BA14 is a fragment of 252P that broke off some time in the past. It's easy to investigate this with AstroGrav by importing these comets using the 'Edit / Import Objects...' command, and then running the simulation backwards in time to see if the comets ever closely approach each other. The accompanying video shows the result, running from the present back to the year 1874.

The distance between the two comets and their minimum orbit intersection distance (MOID) are continually displayed so that they are easily monitored. Note how the minimum orbit intersection distance (MOID) repeatedly jumps from near perihelion to near aphelion during the period 1900 to 1936.

If the simulation is continued back to early 1785, there is a sudden change in the orbit of 252P so that it's perihelion changes from near that of Earth to near that of Mars.

Planetary Eccentricities Over the Last Million Years

Published on 8th March 2016.

AstroGrav screenshot showing the planetary eccentricities over the last million years

AstroGrav contains an undocumented feature that allows you to write objects' data to a file while a simulation evolves. On Windows, it can be invoked with the 'Alt+Shift+Ctrl+E' keystroke followed by evolving the simulation forward or backward. On a Mac, the corresponding keystroke is 'Alt+Shift+Command+E'. The data that is written is in the form of tab-separated text with one row for each time step of the simulation. This can then easily be read, analyzed, and graphed by a spreadsheet application such as Microsoft Excel.

The attached screenshot is an example showing how the eccentricities of orbits of the solar systems' planets have changed with time over the last million years. It was created by running the 'Planets' sample simulation back in time 1,000,000 years with 1,000 year time steps, and then graphing the output with Excel.

Potentially Hazardous Asteroids

Published on 22nd February 2016.

AstroGrav screenshot showing all known potentially hazardous asteroids

With the latest early access release of AstroGrav 3.3, it's now easy to filter huge tables of asteroids to pick out just the ones in the 'Potentially Hazardous' category. Here's an AstroGrav screenshot showing their current positions and orbits. Jupiter is visible on the right, and the four inner planets are highlighted too, but not easy to spot amongst all the asteroids.

Here's a list of instructions for creating such a simulation for yourself, using the latest early access release of AstroGrav 3.3.

  1. Use the 'Open...' command in the 'File' menu to open the 'Planets, Moon' sample simulation.
  2. Choose the 'Import Objects...' command from the 'Edit' menu.
  3. Select 'MPC Unusual Minor Planets' from the displayed list. This will display a table of about 14,000 asteroids.
  4. Select 'Potentially Hazardous' from the 'Category' drop-down menu at the upper-left. This will reduce the table to about 1,630 asteroids.
  5. Select all of them by using the 'Control+A' keystroke ('Command+A' on a Mac).
  6. Press the 'OK' button at the lower-right, and that's it!

You can now manipulate the view windows and evolve the system in the usual way.

The Proposed Ninth Planet of the Solar System

Published on 27th January 2016.

This is an AstroGrav video that shows a simulation of the proposed ninth planet, as detailed in 'Evidence for a Distant Giant Planet in the Solar System' by Konstantin Batygin and Michael E Brown, and featured in many media articles in late January 2016.

The sun is near the centre of the video, with the eight known planets too close to the sun to distinguish. Pluto can just be seen orbiting about four times a second, which gives a good idea of the scale of the other orbits shown. These are the orbits of the Kuiper Belt objects 2012 VP113, 2013 RF98, 2004 VN112, 2010 GB174, 2007 TG422, and Sedna. The stars of the constellations Centaurus, Crux, and Musca can be seen on the left; the stars of the constellations Vela, Carina, and Volans can be seen in the middle; and the stars of the constellations Puppis and Pictor can be seen on the right.

The video is recorded at 24 frames per second with a time step of 40 years between each frame, so the video runs at a speed of about 1,000 years per second. It starts at 15,000 BC and ends at 15,000 AD, with the present time being shortly after the midpoint.

The 2017 Total Eclipse of the Sun

Published on 11th January 2016.

This is an AstroGrav video that shows a simulation of the total eclipse of the sun on 21st August 2017 as seen from the following twelve different viewpoints in the United States of America.

  • Top row, left to right:
    • Seattle in Washington
    • Bismarck in North Dakota
    • Chicago in Illinois
    • Boston in Massachusetts

  • Middle row, left to right:
    • Salem in Oregon
    • Lincoln in Nebraska
    • Nashville in Tennessee
    • Charleston in South Carolina

  • Bottom row, left to right:
    • Los Angeles in California
    • Phoenix in Arizona
    • Albuquerque in New Mexico
    • Houston in Texas

The eclipse is total in the four locations in the middle row. The locations in the top row are all to the north of the line of totality, and the locations in the bottom row are all to the south of the line of totality. Lines of altitude and azimuth can be seen in the background, although you will probably need to pause the video to read them. The video is recorded at 24 frames per second with a time step of 20 seconds between each frame, so the video runs at 24 x 20 = 480 times real-life speed.

The Moon in 2016

Published on 7th January 2016.

This is an AstroGrav video that shows a simulation of the changing the size and phase of the Moon in the year 2016, as seen from the Earth.

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