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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What Was That Bright Light in the Sky Over the Southwestern US on November 7, 2015?

Question: On Nov 7th around 7PM in Tucson, we observed a very unusual sight in the sky, one that I have never seen before.  I believe it might have been related to the missile test off CA, but the way it manifested itself here was very bizarre.  We first observed a large circular light area against the black night sky in the west, probably around 30 degrees in diameter, fairly bright and very distinct as a white glow.  More faintly there was a conical greenish glow, a bit fainter, in the center of this area, looking somewhat like a largish comet.  This appearance did not change for perhaps 20 mins but was stationary in the western sky.  We also noticed just off to the north, a faint white “track”, vertical, as if something had launched into the sky.  It was about 20 degrees in length.  Again, lasted for some time.  I wonder if this isn’t some sort of projection of the events in the Pacific, happening around sundown out there?  Could it be related to the geomagnetic storm?  Did almost have an aurora appearance?  — Tim

Answer: This light in the sky was widely reported by quite a few people.  Apparently it was an unarmed Trident missile that was test-fired from a submarine off the coast of Southern California.

Jeff Mangum

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What was that Bright Light in the Sky?

Question: On September 6, in Glendale AZ, at 20:45 hrs, my husband and I thought we saw a satellite in the North Sky.  (We are sky watchers.)  We agreed it was too bright.  Then it went in circles, then east and west, sometimes fast, slow or still, u-turns, bobbing, even a kind of dancing.  Our neighbor saw it.  Her neighbor saw it Sept 5th. A group of our neighbors said they watched it a month earlier.  I contacted the National UFO Network (NUFORC) and they said it was Vega.  No way.  This has me bugged.  What do you say?  — Kare

Answer: There is really no way to know exactly what it is you saw.  Aircraft would be the most likely answer given the behaviour your note.  Other options require more complex scenarios.  It is often useful to keep Occam’s Razor in mind when evaluating situations such as this.  In the end, it is usually the least complex answer that is the correct answer.

Jeff Mangum

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How to Determine the Relevant Date for a Star Chart

Question: How can I find the date from a given star-chart?  — Anand

Answer: In the following I am going to assume that you are using a star chart appropriate for the mid-latitudes in the northern hemisphere.  The easiest way to determine what date a particular star chart refers to is first know the approximate dates that a few bright stars transit at midnight:

  • Pollux: January 15
  • Regulus: February 22
  • Arcturus: April 25
  • Vega: July 1
  • Deneb: August 1
  • Aldebaran: December 1
  • Capella: December 10

If your star chart has any of these stars near transit then the date for that star chart is likely very close to the relevant date listed above.

Jeff Mangum

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What Caused the Bright Crater I Now See on the Moon?

Question: Sir Took a picture of the Moon a few days ago and in the lower right hand [corner] a crater was very bright. Was the Moon hit recently?  — Bill

Answer: I am not aware of any recent collisions between our Moon and an object that would create a sizeable crater.  Note, though, that existing craters on the Moon can appear brighter or dimmer as a function of how they are illuminated by the Sun.  For example, a crater can appear quite shadowed one day, then a few days later, when the Sun’s illumination angle has changed to be more direct, appear quite a bit brighter.  This may explain your observation.

Jeff Mangum

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Motion of the Stars in the Night Sky

Question: Why do we see the same stars every day of the year in there same location when we orbit?  No way does the Cosmo’s move with us.  — Mike

Answer: In fact, the stars that one can see in the night sky on any given night change as the Earth moves along its orbit around the Sun.  You can test this yourself by noting the location of a bright star on one night at a specific time.  Come back a few weeks later at the same time and note the position of the same bright star.  You will find that the star has moved a bit to the west from its previous location.  This is due to the fact that the Earth has moved a bit along its orbit, which presents a somewhat different view of the stars in our night sky than it did previously.  If you wait an entire year between star position measurements, though, you will find that your star has come back to the same spot.

Jeff Mangum

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