Is the following phrase correct?
I sightsaw London/the museum.
On the other hand, I have never heard anybody actually say it. I have heard plenty of people say went sightseeing. A quick trip to Ngram to compare the usage of sightsaw against went sightseeing comes up with plenty of instances of went sightseeing, but sightsaw is 'not found'.
It appears that although some dictionaries are willing to list it as a verb, real live people don't actually use it as a verb.
It's debatable, and a lot of contributors are going to say no. MW says it is an intransitive verb and lists the past tense as "sightsaw". OALD on the other hand only mentions sightseeing, and that it is a noun. Depends on your dictionary I guess. It is not in common usage now, but language is always evolving, and what is not in usage now may very well be in 50 years.
Actually. it is. My first reaction was "Of course not, sightsee is intransitive", and that is certainly the only way I have ever heard it used. However, Collins Dictionary implies that it is transitive, and the OED (subscription-only, so not linkable) has a secondary transitive meaning with respectable citations, including one from 1968, "I spent the day sight-seeing Berlin.", which seems an almost exact parallel.
So yes you can use the phrase, but I still wouldn't recommend it in a formal (or even non-journalese) context.
Use of the form sightsaw, while grammatically consistent, is nevertheless nonstandard and awkward. Furthermore, if used as a verb, to sightsee is intransitive, so the formulation "sightsaw London" would be incorrect; "sightsaw in London" should be used instead.
The reason why it seems awkward to place to sightsee in the past tense, more so than other verbs, is that the word was not coined as a verb. Rather, it was first coined as a noun, to describe the activity of sightseeing, with a sightseer being one who sees sights. And because of its historical use as a noun, repurposing the word as a verb is novel, and therefore comes across as nonstandard and even awkward.
For this reason, I would recommend caution using to sightsee as a verb in writing, the same as any other nonstandard grammatical construction. The prerogative is yours as a writer, but you should be aware that the usage is nonstandard and will come across that way to your readers.
Now, regarding transitivity, I assert that to sightsee, if used as a verb, must be intransitive (despite dictionary opinions to the contrary), the reason being that the original verb to see is transitive in this construction and already has an object, "sights". Since to see has already discharged its transitivity in the object "sights", the full construction to sightsee cannot anticipate another object, and must therefore be intransitive.
To apply this to the proposed clause "I sightsaw London", "London" cannot be the object of the verb sightsaw because to see has already been satisfied with an object, "sights". The correct formulation of the clause must be "I sightsaw in London", equivalent to "I saw the sights in London".
It may be instructive to compare with other similar constructions in English, such as faultfinding and whale watching, both coined as nouns describing activities.
Supplementing the answers that have been posted, I feel that some clarification is in order.
The expression sightseeing is normally fixed, it is often preceded by the verbs do, be or go.
Q: What are you doing tomorrow?
A: I'm sightseeing
A: I'll be sightseeing
A: I'm doing some sightseeing
A: I'm going sightseeing
A: I'm going to go sightseeing
The structure of go sightseeing is no different from say: go swimming, go dancing, or go fishing, but whereas swimming, dancing and fishing are also verbs, the expression sightseeing is different, it is more like a noun. You do not normally conjugate sightsee
Instead the following is more idiomatic in English
However, sightsee is often conjugated in the past continuous tense, and this structure feels very natural and idiomatic to me.
Lastly, you do not normally go sightseeing in a museum. The verbs visit and see are usually preferred in English.
Q: What did you do yesterday?
1. I visited the British Museum
2. I saw the British Museum
3. I went sightseeing in the British Museum. (RARE)
4. I sightsaw the British Museum (INCORRECT)
To understand how rare sightsaw (blue line) is, compared to went sightseeing (red line) see the Ngram chart below
Although sightsaw has been used very rarely by native speakers, at least in its written form, we cannot dismiss it as being non-existent. Even distinguished British and American authors didn't flinch at using it in their informal correspondence.
The following is an excerpt from the Diaries of James Lees-Milne written between 1942 and 1954
It was their first time on the Continent, and they sightsaw for a week in Paris—daytripping to Versailles, where they visited the Grand Trianon and the Palais. The rest of September, they visited Switzerland and Italy's lakes before sailing home ...
The American writer Allen Ginsberg used the noun as if it were a proper verb in a letter addressed to Robert Creeley in 1967
…then we went on to Paris and sat on Pont des Arts and looked at the summer trees along the seine and sat in cafes and sightsaw, I got hotels and taxis and carried luggage…
As the other answers point out to sightsee sth. as a full-fledged transitive verb is very rare if not generally unacceptable. To me, "sight-seeing" is visiting famous places, doing "touristy" things. For one place only, I would use visit (or saw):
I visited the museum.
I saw Big Ben.
For London, it might be better to paraphrase as
I went sight-seeing in London.
I did some sight-seeing in London.
I spent the day/week-end sight-seeing London.
Your other answers are right in the general case. But you could get away with it in the right register -- not a formal one. I suggest that to go sightseeing somewhere is to see some/most of the sights, while to sightsee -- transitive -- somewhere is to see all the sights. So "I sightsaw London until there were no more sights to see"/"I sightsaw the hell out of London" would be non-conventional but easily understood forms for seeing all the sights.
No. Regardless of a couple of dictionaries, the past tense of 'sightsee' does not see use, and 'sightsee' itself does not see use except in extremely rare circumstances.
The common use is 'sightseeing' as in 'I went sightseeing in [town, city, or area]', or 'saw', as in 'I saw the [more specific landmark or very specific location]'. Also as a response to 'where did you go' 'sightseeing in [town, city, or area]' or 'what did you do?' 'sightseeing in [town, city, or area]'. Sometimes tours will be advertised as 'sightseeing tours', or just 'sightseeing'.
I have never seen or come across 'sightsaw' and google backs up that that phrase/word does not see use whatsoever even amongst poor english speakers.