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Pluto's orbit overlaps Neptune's, does this mean Pluto will hit Neptune sometime?
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26 votes

No, Pluto is a so called resonant trans-neptunian object; the orbital period of Pluto is almost exactly 3:2 (1.5) times that of Neptune. This means that every time Pluto nears perihelion and is ...

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Questions about spiral galaxy arms
21 votes

Actually, the stars and nebulae that make up the spiral arm are only temporarily part of that spiral arm. Spiral arms are more like sound waves where individual particles move around a more or less ...

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Is there any point on earth where the moon stays below the horizon for an extended period of time?
13 votes

Depends on the interpretation of your question... The best places not to observe the moon are the north and south pole. On the north pole you will only be able to see objects above the celestial ...

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Which stars did the Sun form with?
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10 votes

Most stars form in clusters, so it is very likely that the Sun was part of a star cluster when it formed. But in On the Dynamics of Open Clusters, the relaxation time of a cluster is calculated to ...

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Do we still use the term "astronomical unit" nowadays?
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10 votes

Certainly. Astronomical Unit is probably one of the most used distance units used in astronomy. It is of course only used when discussing the distances within a stellar system, such as the distances ...

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What is the angular diameter of Earth as seen from the Moon?
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10 votes

You can calculate the angular diameter of the Earth using the equation: $$a = \arctan \frac{D}{d}$$ where $a$ is the angular diameter, $D$ is the physical diameter of the Earth, and $d$ is the ...

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Is it possible to see a moonrise or moonset twice in a day?
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8 votes

This can only happen if the moonrise at a certain date is earlier than the moonrise at the previous day. There are two reasons why this could happen: The body (Moon, Mars,...) moves in the opposite ...

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Equations for coordinates of solar system objects
6 votes

It depends a bit on how precise you would want to be. A very good discussion on how to calculate the orbits of solar system objects is given in the book by Jean Meeus, Astronomical Algorithms (1999), ...

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Mass, Radius, Colour, Size, Type of a Star from the Hipparcos Catalog
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6 votes

The apparent magnitude (in V ~ visual) is in column 42-46. Other magnitudes are BT magnitude (columns 218-223) and VT magnitude (columns 231-236). These are magnitudes recorded with different (colour) ...

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Is it proper to refer to objects beyond Neptune as "Kuiper Belt Objects?"
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5 votes

Yes there is a difference, Kuiper Belt Objects (KBOs) are a subset of Trans Neptunian Objects (TNOs). Other subsets are Oort-cloud objects (OCOs) and scattered disk objects (SDOs). These are not KBOs ...

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What does designation VY, NML, UU in star names stand for?
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5 votes

VY Canis Majoris and UY Scuti are variable star designations. The first discovered variable star in a constellation is called R, the second, S, and then unto Z. After Z comes RR and so on. For a full ...

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Diameter of any galaxy
5 votes

Given the angular diameter $a$ in radians and the distance $d$ in Mpc, you can get the actual diameter $D$ from: $$D = d\tan{a}$$ Using the small angle approximation, you get: $$D = da$$ $a$ is in ...

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Star class according to initial mass
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5 votes

The classification of stars using spectral class is a very useful classification when considering the properties of (the atmosphere of) a star at that moment. If you consider the different stages in ...

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Is the Sun homogeneous?
5 votes

No it does not have the same composition everywhere. In the core hydrogen is fused into helium, so the fraction of hydrogen (denoted by $X$, between 0 and 1) decreases while the fraction of helium ($Y$...

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Does the gravity of the planets affect the orbit of other planets in our solar system?
5 votes

Depends on what you would call noticeable. The perturbations between the planets are quite small and you will only notice them if you either measure the positions of the planets very accurately or ...

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Calculate the absolute magnitude for a multi-star system
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4 votes

Yes, you are correct. The luminosities can be added. Luminosity is the amount of electromagnetic energy emitted per unit of time (measured in $ \textrm{J} \cdot \textrm{s}^{-1} $ or $\textrm{W}$). So ...

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Equinox sunrise sunset direction
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4 votes

Due to the atmospheric refraction the Sun rises earlier and sets later as you correctly wrote. Refraction however is perpendicular to the horizon. So if the apparent sun rise is earlier, than the true ...

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Why Earth's perihelion occurs 3rd January rather than 1st?
4 votes

The beginning of the year (1st of January) has been chosen for historical reasons (near the winter solstice) and has absolutely nothing to do with Earth's perihelion passage. Earth's perihelion could ...

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What is the farthest point of light that is emitted by a torch?
4 votes

Potentially photons emitted by your torch could travel to infinite distances, assuming the universe will keep on expanding. If the light gets further away, the photons of your torch will be spread out ...

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the most accurate system that can show the positions of planets at my birthday
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4 votes

The most accurate system is the ephemeris service from Jet Propulsion Laboratory (JPL): http://ssd.jpl.nasa.gov/horizons.cgi

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How to measure distances to stars by means of spectroscopic parallaxes?
4 votes

Actually the method described on Wikipedia is not the method that is meant by Spectroscopic Parallax. To determine the spectroscopic parallax, you'll need a spectrum of the star and measure the widths ...

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Motion of rogue planets
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4 votes

1) this question has no real answer as it depends on the reference frame being used. It is very unlikely that they will be stationary except in their own reference frame. rephrased 1) In theory ...

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Computing the Sérsic profile of a galaxy from jpg images
4 votes

Is there any standard software or algorithm for computing the Sérsic index from an image? I don't think it is standard but Vika et al (2013) have used a modified version of GALFIT to extract Sérsic ...

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Transformation from Geocentric Coordinates into Equatorial Coordinates?
3 votes

If I understand your question correctly than your problem is that the primary direction of vCR[3] is the intersection between the prime meridian and the Earth's equator, instead of the vernal equinox. ...

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Astronomical databases for machine learning?
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3 votes

The European Southern Observatory has catalogues with image data available from http://www.eso.org/qi/, you will have to register before you are able to access them. I'd suggest you look at other ...

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Orbital Elements Mars near June 2014
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3 votes

As @barrycarter mentioned you can use the Horizons web interface from JPL: http://ssd.jpl.nasa.gov/horizons.cgi You can change the setting in the web form by clicking on the [change] links for the ...

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How to complete the Hipparcos Catalog?
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3 votes

Most of the stars in the Hipparcos catalogue do not have a common name. In the main catalogue file you will also find the Henry Draper (HD) number of the star (if it has one) at columns 391-396. You ...

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Planets and Pluto? Neptune?
3 votes

There is actually disagreement on this matter (within the IAU?). Dr. Alan Stern (lead of the New Horizons mission) for instance points out that "this rule is inconsistent" (e.g. see Pluto vote '...

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Adaptive Optics?
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3 votes

There are of course books about adaptive optics. For instance: Tyson, R. Principles of Adaptive Optics, (2010).

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Is there a radius inside of which objects are (doppler) blue shifted?
3 votes

No, there is no spherical volume of space within which objects all tend to approach us and thereby show blue-shift. The fact that the Andromeda galaxy is blue-shifted is because at these relatively ...

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