David Hammen
  • Member for 7 years, 3 months
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  • Houston, TX
Is the sun's volume shrinking currently?
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52 votes

Is the sun shrinking currently? It's the other way around: The Sun is slowly growing hotter and thus is slowly expanding. The Sun accumulates ever more helium in its core as it ages. This growing ...

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What is the most accurate measurement of the Astronomical Unit?
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41 votes

The astronomical unit was defined to be exactly 149 597 870 700 meters in 2012, so it is no longer a measured value. In fact, it never was a measured value. It was instead a computed value, and it was ...

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Is it possible that the shadow of The Moon is a single dot during solar eclipse?
40 votes

What you are calling a focal point is the end of the umbra, the point at which the umbra changes to the antumbra. In a total solar eclipse, that point is below rather than above the surface of the ...

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Why is there so little nitrogen in the Martian and Venusian atmospheres?
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36 votes

Nitrogen, with a molecular mass of 28 atomic mass units, is too light to have remained in Mars's atmosphere. Carbon dioxide, with a molecular mass of 44 amu, could (and does) exist on Mars, but it is ...

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How can Io be tidally heated while it is in tidal lock?
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34 votes

How can Io be tidally heated while it is in tidal lock? It is tidally locked in a mean motion sense of "tidally locked". That Io is in an eccentric orbit rather than a circular orbit means ...

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How was the mass of Venus determined?
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32 votes

How was the mass of Venus measured for the first time? In the mid 19th century, Urbain Le Verrier's predicted of the existence of a then unknown planet beyond the orbit of Uranus. He even predicted ...

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Is it a coincidence that on the far side of the moon almost no seas are visible?
31 votes

The current configuration has the Moon's center of mass closer to the Earth than the Moon's center of figure by a couple of kilometers. The reason for that disparity is that the Moon's far side has a ...

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How is it possible that Saturn's gravitational acceleration felt by Mimas is stronger than Mimas' own surface gravity?
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31 votes

An object on Mimas' surface would be much more attracted to Saturn than it is to Mimas. You are missing that Mimas as a whole accelerates gravitationally toward Saturn. What this means is that a ...

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How many times brighter would full-earth-shine appear to people standing on the moon, than full-moon-shine does to people on earth?
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30 votes

How much brighter is full-earth-shine on the moon, than full-moon-shine on earth? On average, it's about 41 times brighter -- in terms of luminosity, or a bit over four times brighter ($2.5 \log_{10} ...

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What will the PRECISE time of the Summer Solstice 2020 be?
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30 votes

TL;DR: In a couple of weeks it might be possible to say precisely (within a second, perhaps a fraction of a second) when the summer solstice did occur. But until then, sub-minute estimates should be ...

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Why can't our Sun be a binary with Jupiter as a T or Y dwarf?
30 votes

But why can't Jupiter be a Y Dwarf who is in the binary relationship with the Sun? There are two reasons: One is that Jupiter is too small to have ever undergone fusion of any sort. To qualify as a ...

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Why do sunspot rise in number "precisely" every eleven years?
28 votes

Why is the periodicity so steady? It's not so steady. Some solar cycles have lasted for nine years, others for fourteen years. Of the 26 solar cycles between 1700 and the present, 21 had a span ...

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Do our sun and moon have names?
28 votes

Of course the Sun and the Moon have names. The names of those objects in English are "the Sun" and "the Moon". Note the use of a definite article ("the") and the ...

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Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?
28 votes

We teach the students: Sidereal day: In 23 h 56 min the earth rotates 360° Solar day: In 24 h the earth rotates 361° You should not teach your students that. You should instead teach your students ...

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Why does the Earth have a z-component at the start of the J2000 epoch?
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28 votes

You made the same fundamental mistake that Anton Gromov made in his question on the sister Space Exploration StackExchange network site: You used the solar system barycenter rather than the Sun as the ...

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Will Earth lose the Moon before the Sun goes into supernova?
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28 votes

As HDE 226868 noted in his answer, the Sun is not going to go supernova. That's something only large stars experience at the end of their main sequence life. Our Sun is a dwarf star. It's not big ...

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Is the Earth-Sun distance 1.012 AU?
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28 votes

As of this moment (2016 May 18, 13:15 UTC), the Earth is 1.0116 astronomical units from the Sun. WA is smart enough to know that "Sun Earth distance in AU" is a time-dependent question. Update: The ...

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What do astronomers do different now that they know about fusion of the Sun?
25 votes

How has knowledge of fusion of the Sun changed astronomy? TL;DR: It changed everything. Up until the 1930s, physicists and astronomers argued for a rather young universe while geologists and ...

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Why does the Parker Solar Probe slow down as the distance from the Sun increases?
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23 votes

Why does the “parker solar probe” lose speed as the distance from the sun increases? Because energy and angular momentum are individually conserved quantities in the two body problem. Except for ...

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Is there any planet/star bigger than VY Canis Majoris?
23 votes

Part 1: Assuming "larger" means greater in diameter. Stars Estimates on the size of stars are just that, estimates, and estimates based on rather fuzzy observations. VY Canis Majoris has been bumped ...

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How do Lagrange points L2 and L3 form?
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22 votes

According to my understanding Lagrange points L1, L4 and L5 can form because gravitation pull can cancel out here as these are between Sun and Earth (where gravitational pull is in two different ...

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Why won't the Sun set for days at N66.2 which is below the arctic circle?
22 votes

Correct me if I am wrong, but if we count sunsets by the center of the Sun apparently crossing the horizon then the Sun is supposed to set every day at latitudes under the arctic circle. That is not ...

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Why would the side of the moon that faces earth be as dark as the far side of the moon?
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22 votes

Somebody suggested that the side of the moon that always faces the earth is as dark as the far side of the moon, but is that really the case? That is not the case. I'll look at two different ...

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How did the Earth come to be in orbit around the sun?
22 votes

The picture was so much cleaner 20 to 25 years ago. I'll present that nice clean picture first. Stars form from the gravitational collapse of huge clouds of interstellar gas. Those gas clouds ...

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The moon has just the right speed not to crash on the Earth or escape into space. What are the odds?
21 votes

My understanding is that the moon was created a long time ago when Earth was hit by a big asteroid. A big asteroid? If you want to call Mars a "big asteroid", then yes, the giant impact ...

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Why are there no terrestrial planets with a subsurface ocean?
21 votes

Some hypothesize that the Earth did have a subsurface ocean during the Cryogenian period, which lasted from 720 to 635 million years ago. The Cryogenian saw the two greatest known ice ages in the ...

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Why can't we see distant galaxies with the naked eye?
21 votes

Surely if you stared long enough, the light from them would eventually hit your eye? Collecting light over a long span of time is how telescopes can see very dim objects. The human visual system ...

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Simple experimental evidence that Earth revolves around Sun
20 votes

You cannot prove that the Earth orbits the Sun rather than vice versa because this goes very much against the grain of all frames of reference being equally valid (but some make a lot more sense than ...

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Why doesn't the sun pull the moon away from earth?
20 votes

Why doesn't the sun pull the moon away from earth? Short answer: Because the Moon is much closer to the Earth than it is to the Sun. This means the gravitational acceleration of the Earth toward the ...

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How can we avoid needing a leap year/second?
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20 votes

Leap years exist for two reasons: There are not an integer number of days in a year. People perceive a need to keep the seasons where they are on the calendar. Given the above, there is no way to ...

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