# Tag Info

## New answers tagged astronomy

0

### Parallax, obliquity, precession, and Orion?

If I understand the original question correctly, it can be formulated as follows: Does ecliptic latitude of a star (its angular distance from ecliptic) change with time? The short answer (known already to Tycho Brahe) is yes, it does. Contrary to what previous comments and answers suggest, ecliptic plane does (significantly) change its position in space ...

3

### Parallax, obliquity, precession, and Orion?

The effect that you're describing is extremely small. Have a look at the following figure: Here you see the position of the Sun from a location $L$ on Earth. Let's call $R_\oplus$ the radius of the Earth, $\Delta$ the distance between the Earth and the Sun, and $\varepsilon$ the obliquity of the Earth. The angle $\delta$ is the declination of the Sun at a ...

6

### Do all known planets and moons have magnetic field?

Our two closest planetary neighbors -- Venus and Mars -- have no significant magnetic fields. In fact, the most recent numbers I know of for an Earth-like (dipole) field on Mars say that its strength as no more than 1/10000th the strength of Earth's. On the other hand, Jupiter's magnetic field is about 20000 times stronger than ours. There is no reason ...

3

### Parallax, obliquity, precession, and Orion?

There would be no effect on the image of Orion due to the obliquity of Earth. Keep in mind that the obliquity you've mentioned here is the angle of Earth's axial tilt with respect to the ecliptic. The angle of the ecliptic is relatively unchanging. While it may be true that certain locations on Earth we may see Orion shifted above or below our equatorial ...

2

### When is the right ascension of the mean sun 0?

No, the right ascension of the mean Sun is NOT zero at the vernal equinox. It is in fact nearly identical to the ecliptic longitude of the mean Sun (the difference is due to UT vs ephemeris time), and this is defined such that it coincides with the ecliptic longitude of the apparent Sun when the Earth is at perihelion. So that should be the starting time to ...

1

### Curiosity Rover (MSL): current coordinates

For the layman interested in coordinates, go here and then click on the map link for any given day: http://curiosityrover.com/tracking/drivelog.html Then you're in the "Google Mars" view, with the rover's path displayed. Of course, you could figure out something to get a list of coordinates in a standard map format. But if you're looking for something ...

1

### Anti-Matter Black Holes

According to Thorne (Black Holes and Time Warps) all matter that approaches the black hole singularity is reduced to a common degenerate form - matter and anti-matter alike. The way I interpret it, matter ceases to retain any resemblance to what existed outside the black hole. The attributes that distinguish matter and anti-matter are stripped away. ...

3

### Distance away from earth to see it as a full disk [duplicate]

How far away from earth's surface do we need to go to see its full hemispherical area? The strict mathematical answer is: no matter how far you go away from earth, you will never see its full hemisphere. So a better question is the one that generalizes your remark "I am interested to know how much of the earth you can see from the International Space ...

2

### Distance away from earth to see it as a full disk [duplicate]

Someone outside a sphere, looking at the sphere, will always see a "full disk"! That's one of the properties of a sphere; it looks like a circle from every direction and distance . The two questions seem to be: 1) How large will that disk appear in the view of the person outside that sphere? Right now, I just looked straight down and noted that the ...

3

### Distance away from earth to see it as a full disk [duplicate]

It seems to me that you have calculated the distance to the horizon rather than the altitude. The altitude would be $\frac{r}{\sin(60^\circ)} - r = r \times 0.1547 = 986\text{ km}$. Update: From within spacecraft the windows would restrict field of view, but in 1966 on gemini-11 mission Richard Gordon did a spacewalk at altitude of 1369km where the ...

1

### Is it possible that universe might not be speeding up expansion?

First of all, galaxies don't shrink. If our own galaxy were shrinking, then we would be moving towards our galactic centre, and we would observe a blueshift in that direction. Second, the accelerated expansion can be determined from the relation between redshift and brightness of distant supernovae. Neither redshift nor brightness would be affected by ...

4

### Is it possible that universe might not be speeding up expansion?

I don't see any logical connection to accelerating expansion. If shrinking of galaxies could explain away the acceleration of the expansion, then it could also explain away the expansion itself. Regardless of whether we're talking about expansion or acceleration of expansion, the effect isn't measured by watching the apparent sizes of galaxies get smaller ...

1

### What planets are visible to the naked eye from Mars?

Aside from having Earth visible in the night sky instead of Mars, you would expect the same planets to be visible. Venus will appear as a bright star close to the sun - smaller than we see it, but still very bright. Jupiter and Saturn will be easier to see in the night sky, and it should be possible to pick out Jupiter's four major moons with the naked ...

1

### Curiosity Rover (MSL): current coordinates

From Twitter: SPICE kernels for my location are available here http://naif.jpl.nasa.gov/naif/. For less technical locations, see https://foursquare.com/marscuriosity NASA is very open. For example, you can get more than 50,000 raw images from the Curiosity mission at the MSL website, and it's updated as they come in.

8

### What happens to the electron companions of cosmic ray protons?

Great question. The electric field creates such a strong force that it would be very hard to move large amounts of just one type of charge. So astrophysical systems do generally eject equal numbers of protons and electrons. In particular, the solar wind is electrically neutral. So these cosmic rays are created in very nearly equal numbers, but by the ...

1

### How do we measure the range of distant objects despite relativistic effects?

Astronomers know about this trouble, and stick with what they can measure. For distant galaxies, quasars, cosmic background radiation, etc. they use only the redshift, the "z" value. This is defined by the measured wavelength and the known laboratory value - assuming any spectral emission and absorption lines are correctly identified. To say anything about ...

0

### How do we measure the range of distant objects despite relativistic effects?

The difficulties are in some ways worse than you imagined and in other ways not as bad. The difficulties are worse in the sense that when we're dealing with distant galaxies, we need general relativity, not just special relativity. General relativity does not even have a well-defined notion of a global frame of reference, so it doesn't offer a uniquely ...

-1

### How do we measure the range of distant objects despite relativistic effects?

The first thing to note is that most accurate distance measurements are at about the 5% level (except for a few exceptions), but 50% is more common - and often satisfactory. Length contraction is not an issue for two reasons. First, relative velocities between us and entire galaxies are generally small1 (especially due to 'peculiar velocities'). Second, ...

-1

### Gravity on the International Space Station

Every object in a stable near-circular orbit around the Earth is actually falling towards the Earth at an accelerating speed or at an ever-increasing falling-speed, but because of its horizontal speed around the Earth, ten times faster than a rifle bullet for the ISS, the surface of the Earth curves away below it at the same falling-speed, and it therefore ...

12

### Why don't we see solar and lunar eclipses often?

Just to put you at ease first, this is not an infantile question. The reason we do not have a solar eclipse at every new moon is mostly due to the angle of Earth's axis (and by extension, the Moon's orbital plane) to the Earth-Sun line. See the picture below for a visual explanation. In the picture, the Sun is to the left. The upper image shows the orbit of ...

4

### Why wasn't the moon visible during the day a few decades ago?

Believe it or not, the Moon was visible during the day in 1949. In fact, the Moon has always been visible during the day at certain parts of the lunar cycle. We know this is true not only because of models of the Earth-Moon system, but there is historical evidence of it! There are records dating back to ancient China in 2800 BCE of solar eclipses, which are ...

0

### Why wasn't the moon visible during the day a few decades ago?

50 years is nothing on an astronomical scale, so my guess is: you just didn't notice it/pay attention to it/don't remember correctly.

3

### Why wasn't the moon visible during the day a few decades ago?

During daylight, you can only see the moon when the sun is fairly low in the sky and in the correct phase. For example, about 1 week after full moon you should be able to see it in the morning when the sun is not very high in the eastern sky. At that time, the 1/2 moon should be quite visible between vertically and into the western sky. Similarly, about 1 ...

1

### Alpha Centauri Bb: Comparing astrometric precision vs doppler precision

Ok, I know this question was posted some time ago, but I just found it using Google because I asked myself a similar thing. First of all: No, Gaia won't be able to detect Centauri Bb. But the astrometric method has one strange property: It get's better if the planet is farther away from its host star (as long as the observational time needed doesn't get too ...

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