Where Can I Find Transit Times for Astronomical Objects?

Question: Is there a way or a program available that will tell me what point on earth is 90° below it. For example for the sun and moon they show when the sun is 90° above a certain location. Can I find out the same thing for mercury and venus?  — Steve

Answer: I think what you are asking for are the times when astronomical objects are at their highest point in the sky, otherwise known as “transit”.  The Naval Observatory Data Services provides a nice web page which includes links to quite a few calculators which allow one to determine rise, set, and transit times for many astronomical objects.

Jeff Mangum

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Why Does a Small Change in Height Cause the Full Moon to be Occulted by a Nearby Mountain?

Question: I cannot find an answer to this conundrum anywhere on the Web, and it’s not at all a function of the famous Moon Illusion.  Late last month (March), the full moon rose over a 7500-foot elevation ridgeline that’s located about 10 miles from my house patio viewing spot at 3500-foot elevation.  While standing up I watched the top half of the moon clear the ridgeline, but upon sitting down in a chair I noticed the view of the moon had been reduced to a mere sliver.
I understand that refraction considerations might play into this, but really?  to that great a degree?  Essentially, the effect of a 3-foot vertical difference in my viewing height resulted in half the rising moon being obscured by the ridgeline.  However, a tiny 3-foot difference distributed over 10 miles of horizontal distance seems to me as if it’d be way too small to result in any appreciable change of moon-viewing angle/perspective.  What else could I be missing here?   — Rick

Answer: I believe what you observe is due to your proximity to the mountain that is approximately 4000 feet above your point of observation and not due to refraction.  Now, to cause the Moon to go from half-blocked to only slightly blocked by a nearby mountain, I would think that you would need to be closer than 10 miles, though.

Jeff Mangum

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Does the Prime Meridian of Mars Ever Parallel to the Celestial Prime Meridian?

Question: Is there ever day in Mars history where the Mars prime meridian is parallel to the prime meridian on the celestial sphere? If so what is it?  For example on earth on 1950-3-21T04:35-21 spring equinox the earth prime meridian is parallel to the celestial sphere prime meridian.  I am wondering if that  ever happens with Mars and what that date would be if it ever happen. I don’t think its as easy as just saying spring equinox on Mars means the same thing.  — Steve

Answer: This is kind of a tricky question as the celestial meridian is defined as the great circle passing through the celestial poles, the zenith, and the nadir of a particular location.  So, if you are standing on Mars the prime meridian of Mars will be exactly parallel to the celestial meridian twice per year, once during the Mars spring equinox then again during its autumnal equinox.  Note that the two meridians cross (though are not strictly parallel) once each day.

Jeff Mangum

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Will a Comet Skim the Earth’s Atmosphere in 2016?

Question: I’ve been dealing with a false prophet who says that a comet is coming and is going to skim the earth, as if to skip off of it, like a stone skipping on water.  Is this even possible?  She says it will skip off of the earth and keep going into space.  Please let me know if this is even possible?  Thanks so much.  — Andrew

Answer: The scenario you describe is physically possible, but the actual event described by this “false prophet” is misrepresented.  A pair of comets, known as 252P/LINEAR and P/2016 BA14, passed close to Earth on March 21 and 22, 2016.  The closest that they came was within 2.2 million miles of the Earth, or about 9.6 times the distance between the Earth and the Moon.  Comets have on a couple of occasions passed closer to Earth, and as was the case with the most recent comet passage we survived.

Jeff Mangum

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Is the Southern Cross Visible from Panama?

Question: We will be visiting Panama from April 2-8.  Will we be able to see the southern cross and where and at what time should we look for it.  Thanks!  — Cheryl

Answer: You should be able to see the Southern Cross (the constellation Crux) in the southern sky from any location in Panama rising around 9PM, transiting around 1AM, and setting around 5AM local time.  If you are looking for it when it transits just look south at about 25 degrees above the horizon and you should have no trouble spotting it.

Jeff Mangum

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Why Do High Tides Happen Both on the Side of the Earth Facing Toward and Away from the Moon?

Question: During new moon, moon is between earth and sun. The spring tide then occurs. The height of high tide is higher. But on right side of image why is water having high tide. As there is no force pulling water from right side of image. As sun and moon are on left.  — Priyanshu

Answer: You are right in that high tide occurs on the sides of the Earth which face toward and away from the Moon.  This is due to the fact that around the Moon is pulling on the Earth, and the ocean, on the sides facing the Moon.  The Earth compensates for this pulling by bulging out both toward and away from the Moon.  This results in more water being displaced in these directions, resulting in high tide.  One gets the higher tides, called spring tides, when the gravitational force of the Sun is added to that of the Moon, which makes the bulge on the Earth’s surface a bit larger than that caused by the Moon alone.  These spring tides happen around New and Full Moon.

Jeff Mangum

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Do We Observe the Sunrise Six Minutes After it Actually Occurs?

Question: We know sunlight takes about 8 min to reach earth. So we see sunrise 8 min later. Also we see sunrise 2 min earlier due to atmospheric refraction. Does it mean that we see sunrise after about 6 min of actual sunrise??  — Priyanshu

Answer: I think that you are right, in the sense that if the Sun suddenly turned-off at sunrise one day, we would not notice this until about 8 minutes (of light travel time) minus 2 minutes (due to atmospheric refraction allowing us to see the Sun a bit below the horizon), or 6 minutes.  The refraction bit amounts to about 34 arcminutes of bending of the light from the Sun, which is about 2 minutes of time.  Note, though, that this ignores the difference between the definition of sunrise, which refers to the point at which the upper limb of the Sun is at the horizon, and the position of the Sun, which is referenced to its center.  If you add the half-size of the Sun, or about 15 arcminutes, then sunrise occurs when its center is about 50 arcminutes below the horizon, which equates to about 3 minutes of time.

Jeff Mangum

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How Can One Tell What Telescope Made an Image of a Deep Space Object?

Question: When looking at an image of a deep space object,  how can you tell what type of telescope was used to take the photo?  — Jeni

Answer: Unless the caption or other description of the image lists the telescope used to make the image, it is often hard to tell which telescope was used to make an image of an astronomical object.  Fortunately, at least in my experience, the origin of an astronomical image is almost always listed with the information associated with the image.

Jeff Mangum

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Why are Telescopes Better at Observing Astronomical Objects than the Naked Eye?

Question: Why are telescopes better than the naked eye?  — Amanda

Answer: Well, in fact, there are circumstances when the naked eye is a better way to observe astronomical phenomena than a telescope.  For example, if you are interested in viewing the general structure of the night sky, such as constellations or the dust lanes what comprise the Milky Way, you are better off using just your eyes, as a telescope would restrict you to observing just a small region of the sky.  Telescopes, on the other hand, are best for getting a very close-up look at the details of most astronomical objects, including planets, stars, and galaxies.  Telescopes allow you to see details, due to their higher spatial resolution, than one can see with the naked eye.

Jeff Mangum

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Why Are the Best Times to Observe Meteors Between Midnight and Dawn?

Question: Why are all the best times for viewing meteor showers between midnight and dawn?  As I imagine looking down on the solar system from above, I think of a stream of comet debris as impacting all of our outward-facing atmosphere (away from the sun) uniformly at the same time.  Why isn’t it as good as soon as it becomes dark?  — Steve

Answer: The main reason that the best viewing times are between midnight and dawn is due to the orientation between your location on the Earth’s surface and the direction of travel of the Earth in its orbit.  Starting around midnight your location on the Earth’s surface spins around to the forward-facing half of Earth, relative to the direction that the Earth is travelling in its orbit around the Sun. At dawn, your location on the Earth’s surface is pointed straight in the direction of the Earth’s travel in its orbit.  Therefore, between midnight and dawn you are moving head-on through the location of the meteors in space, which means that you will, on average, observe more of them.

Jeff Mangum

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