<blockquote>
  <p>Is there a more rigorous definition of these three terms that distinguishes an eclipse from an occultation or a transit?</p>
</blockquote>

<p>Not quite rigorous, but</p>

<ul>
<li>Occultations refer to events where the intervening object's angular size is significant larger than is that of the more remote object,</li>
<li>Transits refer to events where the intervening object's angular size is significantly smaller than is that of the more remote object, and</li>
<li>Eclipses cover the fuzzy region where the angular sizes of the remote and intervening object are roughly equal.</li>
</ul>

<p>The boundary is fuzzy. The Earth's angular diameter is 33 times larger than is that of the Sun from geosynchronous altitude, or 1.5 orders of magnitude larger. Yet the periods of time where geosynchronous satellites can be in the Earth's shadow are called "eclipse season" rather than "occultation season". On the flip side, Venus's angular diameter is 33 times smaller than is that of the Sun when Venus crosses in front of the Sun, and those events are most definitely called transits.</p>

CC BY-SA 4.0
2018-08-24T16:12:33.680Z
@barrycarter - From the perspective of the Moon, the Earth's angular diameter is about 3.67 times larger than is the Sun's, or half an order of magnitude. Whether that qualifies as "significantly larger" is dubious. (It's a lot less dubious than calling events where the Earth blocks the view of the Sun from the perspective of a geostationary satellite an "eclipse".)
David Hammen
CC BY-SA 4.0
2019-08-22T23:18:03.137Z
This is interesting, I wonder how it applies to [mutual occultations and eclipses of the Galilean satellites](https://astronomy.stackexchange.com/a/23483/7982)
uhoh
CC BY-SA 4.0
2018-08-24T10:13:02.203Z
2018-08-24T10:13:02.203Z
David Hammen
<p>There are a number of terms for these things:</p>

<p><strong><em>conjunction</em></strong>:<br>
When two objects have either the same right ascension or the same ecliptic longitude (usually as observed from Earth). That is, they lie in (almost) a straight line from the point of view of the observer. In the case of the Solar System, such as when the Sun and Venus line up from the point of view of an observer on Earth, this is a conjunction. (It is also a <em>conjunction</em> when Earth lines up between e.g. Mars and the Sun - it is an <em>inferior conjunction</em> from the point of view of Mars (but we can't observe from Mars)).</p>

<p>Furthermore, when Venus lines up between the Earth and the Sun this is an <em>inferior conjunction</em>, and when Venus lines up with the Sun but on the opposite side of the Sun then this is a <em>superior conjunction</em>. Thus the inner planets (Mercury, Venus) can have both <em>inferior conjunctions</em> and <em>superior conjunctions</em> with Earth while the outer planets (Mars, Jupiter, etc.) can only have <em>superior conjunctions</em> with Earth.</p>

<p>The "<em>almost</em>" in "<em>almost a straight line</em>" is important because the definition is by longitude. Although the planets are said to lie in the ecliptic plane, this is not completely flat - they do not all have the exact same orbit inclinations. Thus not every <em>conjunction</em> is a <em>transit</em>, <em>occultation</em> or <em>eclipse</em> - they do not pass <em>directly</em> in front or behind.</p>

<p><strong><em>transit</em></strong>:<br>
When an object passes in front of a larger object*, this is a <em>transit</em>. These are rarer than <em>conjunctions</em> because the objects need to be directly in line so that one can be seen crossing the other. </p>

<p>In the case of recent transit of Venus, when Venus passed in front of the Sun, the sunlight passing through Venus' atmosphere gave us the opportunity to collect data on its composition. When one of the moons of Jupiter pass in front of Jupiter itself* this is also a transit.</p>

<p><strong><em>occultation</em></strong>:<br>
When an object passes in front of a smaller object*, this is an <em>occultation</em>. The farther object is hidden by the nearer object. These are also rare** because, like <em>transits</em>, they also need to be directly in line so that the nearer object can be seen crossing in front of, and blocking out, the farther object. </p>

<p>Asteroids <em>occulting</em> stars are studied because while we can calculate the asteroid's orbit and its distance from us using its speed, knowing how gravity and orbits work, etc., we can't see clearly enough to determine the size (think few kilometres in diameter but millions of kilometres away). But when an asteroid passes in front of (<em>occults</em>) a distant star, the star is effectively a point source (looks tiny), from the amount of time the <em>occultation</em> lasts and the asteroid's speed we can calculate its size***.</p>

<p><strong><em>eclipse</em></strong>:<br>
The Moon passing in front of the Sun is, depending on the conditions, a <em>partial eclipse</em>, <em>total eclipse</em> or <em>annular eclipse</em>.</p>

<p>Clearly, by the definition, when the Moon fully blocks the Sun it is an <em>occultation</em> and when it only partially blocks the Sun it is a <em>transit</em>, but 
this is a special case and is called a <em>solar eclipse</em> rather than a <em>transit</em> or <em>occultation</em>. (Similarly when the Moon passes through the Earth's shadow - when the Earth is directly between the Sun and the Moon - it is called a <em>lunar eclipse</em>.)</p>

<p>The word <em>eclipse</em> comes to us via Latin and French from ancient Greek <em>ekleipsis</em>, from <em>ekleipein</em> 'fail to appear, be eclipsed', from <em>ek</em> 'out' + <em>leipein</em> 'to leave'. (see <a href="https://en.wikipedia.org/wiki/Eclipse#Etymology" rel="nofollow noreferrer">eclipse on Wikipedia</a>)</p>

<p>This alignment is also known as a <em>syzygy</em>****.</p>

<p>* Smaller or larger as they appear to the observer, an asteroid is most definitely smaller than a star but because it is so much nearer it looks bigger.<br>
** A lookup of <em>occultations</em> can return a long list so that it may not seem rare, but when you compare how many asteroids there are, and how many stars, in relative terms it actually does not happen very often.<br>
*** We can set limits on its size, given that the "average" asteroid looks like a lumpy potato rather than being some nice, regular shape that would make it easy to work these things out.<br>
**** We could keep adding terms and googling them all day... ;-)<br>
***** There is no footnote with five *</p>

CC BY-SA 4.0
2018-08-24T14:25:11.387Z
The Moon also occults stars, and these occur regularly enough to say that they are "frequent".
JohnHoltz
CC BY-SA 4.0
2022-04-02T06:41:39.617Z
I asked this question a long time ago and then completely forgot about it. There's more here than I can possibly hope to retain, but it does answer the question. Thanks for you efforts!
CatchAsCatchCan
CC BY-SA 4.0
2018-08-24T07:43:59.357Z
2018-08-24T12:45:30.137Z
2018-08-24T12:45:30.137Z
Mick
<p>A comment in response to <a href="https://astronomy.stackexchange.com/questions/27432/can-the-moon-eclipse-venus">this question</a> suggests that an eclipse differs from an occultation in that the former casts a shadow while the latter doesn't. </p>

<p>This isn't particularly satisfactory since blocking the light from one object with a second will always cast a shadow even if it's not visible to the human eye.</p>

<p>Similarly, an annular eclipse of the sun by the moon could be referred to as a transit.</p>

<p>Is there a more rigorous definition of these three terms that distinguishes an eclipse from an occultation or a transit? </p>

CC BY-SA 4.0
2018-08-24T04:21:42.060Z
*blocking the light from one object with a second will always cast a shadow* - Not if the second object emits or reflects light.
Lucian
CC BY-SA 4.0
2018-08-25T06:01:16.283Z
@barrycarter: When a planet or moon occults another planet or moon, no shadow is cast, since both the *occulted* body, as well as the *occulting* body, reflect the light of the main star.
Lucian
CC BY-SA 4.0
2022-06-03T23:53:05.683Z
Wikipedia gives the definition here: https://en.wikipedia.org/wiki/Eclipse#:~:text=An%20eclipse%20is%20the%20result%20of%20either%20an%20occultation%20(completely%20hidden)%20or%20a%20transit%20(partially%20hidden).
Cheng
<solar-eclipse><terminology><planetary-transits><occultation>
How does an eclipse differ from an occultation?
CatchAsCatchCan
# How does an eclipse differ from an occultation?

(Asked by: username_0 on 2018-08-24)

A comment in response to [this question](https://astronomy.stackexchange.com/questions/27432/can-the-moon-eclipse-venus) suggests that an eclipse differs from an occultation in that the former casts a shadow while the latter doesn't.

This isn't particularly satisfactory since blocking the light from one object with a second will always cast a shadow even if it's not visible to the human eye.

Similarly, an annular eclipse of the sun by the moon could be referred to as a transit.

Is there a more rigorous definition of these three terms that distinguishes an eclipse from an occultation or a transit?

**Tags:** solar-eclipse, terminology, planetary-transits, occultation

> **Comments:**
> **username_1**: *blocking the light from one object with a second will always cast a shadow* - Not if the second object emits or reflects light.  
> **username_1**: @barrycarter: When a planet or moon occults another planet or moon, no shadow is cast, since both the *occulted* body, as well as the *occulting* body, reflect the light of the main star.  
> **username_2**: Wikipedia gives the definition here: https://en.wikipedia.org/wiki/Eclipse#:~:text=An%20eclipse%20is%20the%20result%20of%20either%20an%20occultation%20(completely%20hidden)%20or%20a%20transit%20(partially%20hidden).  

---

## 2 answers

### Answer by username_5 on 2018-08-24. Score: 6 | Accepted answer

There are a number of terms for these things:

***conjunction***:  
When two objects have either the same right ascension or the same ecliptic longitude (usually as observed from Earth). That is, they lie in (almost) a straight line from the point of view of the observer. In the case of the Solar System, such as when the Sun and Venus line up from the point of view of an observer on Earth, this is a conjunction. (It is also a *conjunction* when Earth lines up between e.g. Mars and the Sun - it is an *inferior conjunction* from the point of view of Mars (but we can't observe from Mars)).

Furthermore, when Venus lines up between the Earth and the Sun this is an *inferior conjunction*, and when Venus lines up with the Sun but on the opposite side of the Sun then this is a *superior conjunction*. Thus the inner planets (Mercury, Venus) can have both *inferior conjunctions* and *superior conjunctions* with Earth while the outer planets (Mars, Jupiter, etc.) can only have *superior conjunctions* with Earth.

The "*almost*" in "*almost a straight line*" is important because the definition is by longitude. Although the planets are said to lie in the ecliptic plane, this is not completely flat - they do not all have the exact same orbit inclinations. Thus not every *conjunction* is a *transit*, *occultation* or *eclipse* - they do not pass *directly* in front or behind.

***transit***:  
When an object passes in front of a larger object\*, this is a *transit*. These are rarer than *conjunctions* because the objects need to be directly in line so that one can be seen crossing the other.

In the case of recent transit of Venus, when Venus passed in front of the Sun, the sunlight passing through Venus' atmosphere gave us the opportunity to collect data on its composition. When one of the moons of Jupiter pass in front of Jupiter itself\* this is also a transit.

***occultation***:  
When an object passes in front of a smaller object\*, this is an *occultation*. The farther object is hidden by the nearer object. These are also rare\*\* because, like *transits*, they also need to be directly in line so that the nearer object can be seen crossing in front of, and blocking out, the farther object.

Asteroids *occulting* stars are studied because while we can calculate the asteroid's orbit and its distance from us using its speed, knowing how gravity and orbits work, etc., we can't see clearly enough to determine the size (think few kilometres in diameter but millions of kilometres away). But when an asteroid passes in front of (*occults*) a distant star, the star is effectively a point source (looks tiny), from the amount of time the *occultation* lasts and the asteroid's speed we can calculate its size\*\*\*.

***eclipse***:  
The Moon passing in front of the Sun is, depending on the conditions, a *partial eclipse*, *total eclipse* or *annular eclipse*.

Clearly, by the definition, when the Moon fully blocks the Sun it is an *occultation* and when it only partially blocks the Sun it is a *transit*, but
this is a special case and is called a *solar eclipse* rather than a *transit* or *occultation*. (Similarly when the Moon passes through the Earth's shadow - when the Earth is directly between the Sun and the Moon - it is called a *lunar eclipse*.)

The word *eclipse* comes to us via Latin and French from ancient Greek *ekleipsis*, from *ekleipein* 'fail to appear, be eclipsed', from *ek* 'out' + *leipein* 'to leave'. (see [eclipse on Wikipedia](https://en.wikipedia.org/wiki/Eclipse#Etymology))

This alignment is also known as a *syzygy*\*\*\*\*.

\* Smaller or larger as they appear to the observer, an asteroid is most definitely smaller than a star but because it is so much nearer it looks bigger.  
\*\* A lookup of *occultations* can return a long list so that it may not seem rare, but when you compare how many asteroids there are, and how many stars, in relative terms it actually does not happen very often.  
\*\*\* We can set limits on its size, given that the "average" asteroid looks like a lumpy potato rather than being some nice, regular shape that would make it easy to work these things out.  
\*\*\*\* We could keep adding terms and googling them all day... ;-)  
\*\*\*\*\* There is no footnote with five \*

> **Comments:**
> **username_6**: The Moon also occults stars, and these occur regularly enough to say that they are "frequent".  
> **username_0**: I asked this question a long time ago and then completely forgot about it. There's more here than I can possibly hope to retain, but it does answer the question. Thanks for you efforts!  

---

### Answer by username_3 on 2018-08-24. Score: 4

> Is there a more rigorous definition of these three terms that distinguishes an eclipse from an occultation or a transit?

Not quite rigorous, but

* Occultations refer to events where the intervening object's angular size is significant larger than is that of the more remote object,
* Transits refer to events where the intervening object's angular size is significantly smaller than is that of the more remote object, and
* Eclipses cover the fuzzy region where the angular sizes of the remote and intervening object are roughly equal.

The boundary is fuzzy. The Earth's angular diameter is 33 times larger than is that of the Sun from geosynchronous altitude, or 1.5 orders of magnitude larger. Yet the periods of time where geosynchronous satellites can be in the Earth's shadow are called "eclipse season" rather than "occultation season". On the flip side, Venus's angular diameter is 33 times smaller than is that of the Sun when Venus crosses in front of the Sun, and those events are most definitely called transits.

> **Comments:**
> **username_3**: @barrycarter - From the perspective of the Moon, the Earth's angular diameter is about 3.67 times larger than is the Sun's, or half an order of magnitude. Whether that qualifies as "significantly larger" is dubious. (It's a lot less dubious than calling events where the Earth blocks the view of the Sun from the perspective of a geostationary satellite an "eclipse".)  
> **username_4**: This is interesting, I wonder how it applies to [mutual occultations and eclipses of the Galilean satellites](https://astronomy.stackexchange.com/a/23483/7982)  

---

